diff --git a/CHANGELOG.md b/CHANGELOG.md
index 1e725f7..e12f67b 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -5,10 +5,11 @@
 
 * QUIC protocol check
 * TLS 1.3 early data (0-RTT)
-* Support for RFC 8998 and draft-yang-tls-hybrid-sm2-mlkem (TLS_SM4_GCM_SM3, TLS_SM4_CCM_SM3 ciphers, kx groups curveSM2, curveSM2MLKEM768; SM2 pub keys + signatures)
+* Support for RFC 8998,  draft-yang-tls-hybrid-sm2-mlkem (TLS_SM4_GCM_SM3, TLS_SM4_CCM_SM3 ciphers, kx groups curveSM2, curveSM2MLKEM768; SM2 pub keys + signatures)
 * Adds a check for mandatory extended master secret TLS extension
 * Bump SSLlabs rating guide to 2009r
 * Check for Opossum vulnerability
+* `--phone-out` checks the HSTS preload list on https://hstspreload.org/
 * Enable IPv6 automagically, i.e. if target via IPv6 is reachable just (also) scan it
 * Provide an FAQ
 
diff --git a/CREDITS.md b/CREDITS.md
index b901810..f4ca94b 100644
--- a/CREDITS.md
+++ b/CREDITS.md
@@ -11,7 +11,7 @@ Full contribution, see git log.
   - extended parsing of TLS ServerHello messages
   - TLS 1.3 support (final and pre-final) with needed en/decryption
   - add several TLS extensions
-  - Several ciphers and curves added 
+  - Several ciphers and curves added
   - Detection + output of multiple certificates
   - several cleanups of server certificate related stuff
   - testssl.sh -e/-E: testing with a mixture of openssl + sockets
@@ -55,6 +55,7 @@ Full contribution, see git log.
   - maximum certificate lifespan of 398 days
   - ssl renegotiation amount variable
   - custom http request headers
+  - HSTS preload list lookup (finalized: Mayank)
 
 * Frank Breedijk
   - Detection of insecure redirects
diff --git a/doc/testssl.1 b/doc/testssl.1
index 821a58b..f0a0a58 100644
--- a/doc/testssl.1
+++ b/doc/testssl.1
@@ -417,7 +417,8 @@ can try to apply evasion techniques by changing the variables USLEEP_SND
 and / or USLEEP_REC and maybe MAX_WAITSOCK.
 .PP
 \f[CR]\-\-phone\-out\f[R] Checking for revoked certificates via CRL and
-OCSP is not done per default.
+OCSP, as well as the HSTS preload list status via hstspreload.org, is
+not done per default.
 This switch instructs testssl.sh to query external \(en in a sense of
 the current run \(en URIs.
 By using this switch you acknowledge that the check might have privacy
@@ -603,6 +604,10 @@ by detection or by enforcing via \f[CR]\-\-assume\-http\f[R].
 It tests several HTTP headers like
 .IP \(bu 2
 HTTP Strict Transport Security (HSTS)
+.RS 2
+.IP \(bu 2
+HSTS preload list status (when \f[CR]\-\-phone\-out\f[R] supplied)
+.RE
 .IP \(bu 2
 HTTP Public Key Pinning (HPKP)
 .IP \(bu 2
diff --git a/doc/testssl.1.html b/doc/testssl.1.html
index 7cfcb82..347a120 100644
--- a/doc/testssl.1.html
+++ b/doc/testssl.1.html
@@ -396,7 +396,8 @@
         evasion techniques by changing the variables USLEEP_SND and / or
         USLEEP_REC and maybe MAX_WAITSOCK.</p>
         <p><code>--phone-out</code> Checking for revoked certificates
-        via CRL and OCSP is not done per default. This switch instructs
+        via CRL and OCSP, as well as the HSTS preload list status via
+        hstspreload.org, is not done per default. This switch instructs
         testssl.sh to query external – in a sense of the current run –
         URIs. By using this switch you acknowledge that the check might
         have privacy issues, a download of several megabytes (CRL file)
@@ -548,7 +549,11 @@
         <code>--assume-http</code>. It tests several HTTP headers
         like</p>
         <ul>
-        <li>HTTP Strict Transport Security (HSTS)</li>
+        <li>HTTP Strict Transport Security (HSTS)
+        <ul>
+        <li>HSTS preload list status (when <code>--phone-out</code>
+        supplied)</li>
+        </ul></li>
         <li>HTTP Public Key Pinning (HPKP)</li>
         <li>Server banner</li>
         <li>HTTP date+time</li>
diff --git a/doc/testssl.1.md b/doc/testssl.1.md
index 7e22d5d..db58e8e 100644
--- a/doc/testssl.1.md
+++ b/doc/testssl.1.md
@@ -152,7 +152,7 @@ The same can be achieved by setting the environment variable `WARNINGS`.
 
 `--ids-friendly` is a switch which may help to get a scan finished which otherwise would be blocked by a server side IDS. This switch skips tests for the following vulnerabilities: Heartbleed, CCS Injection, Ticketbleed and ROBOT. The environment variable OFFENSIVE set to false will achieve the same result. Please be advised that as an alternative or as a general approach you can try to apply evasion techniques by changing the variables USLEEP_SND and / or USLEEP_REC and maybe MAX_WAITSOCK.
 
-`--phone-out` Checking for revoked certificates via CRL and OCSP is not done per default. This switch instructs testssl.sh to query external -- in a sense of the current run -- URIs. By using this switch you acknowledge that the check might have privacy issues, a download of several megabytes (CRL file) may happen and there may be network connectivity problems while contacting the endpoint which testssl.sh doesn't handle. PHONE_OUT is the environment variable for this which needs to be set to true if you want this.
+`--phone-out` Checking for revoked certificates via CRL and OCSP, as well as the HSTS preload list status via hstspreload.org, is not done per default. This switch instructs testssl.sh to query external -- in a sense of the current run -- URIs. By using this switch you acknowledge that the check might have privacy issues, a download of several megabytes (CRL file) may happen and there may be network connectivity problems while contacting the endpoint which testssl.sh doesn't handle. PHONE_OUT is the environment variable for this which needs to be set to true if you want this.
 
 `--add-ca <CAfile>` enables you to add your own CA(s) in PEM format for trust chain checks. `CAfile` can be a directory containing files with a \.pem extension, a single file or multiple files as a comma separated list of root CAs. Internally they will be added during runtime to all CA stores. This is (only) useful for internal hosts whose certificates are issued by internal CAs. Alternatively ADDTL_CA_FILES is the environment variable for this.
 
@@ -213,6 +213,7 @@ Also for multiple server certificates are being checked for as well as for the c
 `-h, --header, --headers`       if the service is HTTP (either by detection or by enforcing via `--assume-http`. It tests several HTTP headers like
 
 * HTTP Strict Transport Security (HSTS)
+    - HSTS preload list status (when `--phone-out` supplied)
 * HTTP Public Key Pinning (HPKP)
 * Server banner
 * HTTP date+time
diff --git a/t/53_hsts_preload.t b/t/53_hsts_preload.t
new file mode 100755
index 0000000..66a5552
--- /dev/null
+++ b/t/53_hsts_preload.t
@@ -0,0 +1,52 @@
+#!/usr/bin/env perl
+
+# Check the HSTS preload list status against the hstspreload.org API (needs --phone-out).
+# github.com is on the preload list, example.com is not.
+#
+# We don't use a full run, only the HTTP header section.
+
+use strict;
+use Test::More;
+
+my $tests = 0;
+my $prg="./testssl.sh";
+my $csv="tmp.csv";
+my $cat_csv="";
+my $check2run="-q --color 0 --phone-out --ip=one --headers --csvfile $csv";
+my $uri="github.com";
+my @args="";
+
+die "Unable to open $prg" unless -f $prg;
+
+# Provide proper start conditions
+unlink $csv;
+
+#1 run -- a domain which is on the HSTS preload list
+printf "\n%s\n", "Unit test for HSTS preload list status against \"$uri\"";
+@args="$prg $check2run $uri >/dev/null";
+system("@args") == 0
+     or die ("FAILED: \"@args\" ");
+$cat_csv=`cat $csv`;
+
+# github.com is on the preload list
+like($cat_csv, qr/"HSTS_preloadAPI".*"preloaded"/,"\"$uri\" should be on the HSTS preload list");
+$tests++;
+unlink $csv;
+
+#2 run -- a domain which is NOT on the HSTS preload list
+$uri="example.com";
+@args="$prg $check2run $uri >/dev/null";
+system("@args") == 0
+     or die ("FAILED: \"@args\" ");
+$cat_csv=`cat $csv`;
+
+# example.com is not on the preload list
+like($cat_csv, qr/"HSTS_preloadAPI".*"no entry"/,"\"$uri\" should not be on the HSTS preload list");
+$tests++;
+unlink $csv;
+
+done_testing($tests);
+printf "\n";
+
+
+# vim:ts=5:sw=5:expandtab
diff --git a/testssl.sh b/testssl.sh
index 97c7af9..69980df 100755
--- a/testssl.sh
+++ b/testssl.sh
@@ -2262,6 +2262,76 @@ check_revocation_ocsp() {
      fi
 }
 
+# Checks a domain against the hstspreload.org HSTS preload list API (requires --phone-out).
+# arg1: domain to check
+# arg2: JSON key to check (e.g. status, bulk, preloadedDomain). Empty: only (re)fetch the response.
+# arg3: value the key is expected to have (without surrounding quotes; quoting is handled here)
+# Return values:
+#      0 - request made, nothing compared (no key supplied)
+#      1 - API request failed (connection error)
+#     10 - key matched the expected value
+#     20 - key present but value did not match
+#     21 - key not found in the response
+#
+check_hsts_preloadlist_match() {
+     local domain="$1"
+     local key="$2"
+     local value="$3"
+     local response=""
+     local tmpfile="$TEMPDIR/$NODE.hsts-preloadlist.txt"
+     local uri_api_status="https://hstspreload.org/api/v2/status?domain=$domain"
+
+     "$PHONE_OUT" || return 0
+
+     # Only query the API once per host, then reuse the cached response
+     if [[ ! -f "$tmpfile" ]]; then
+          http_get "$uri_api_status" "$tmpfile" || return 1
+     fi
+     response="$(<"$tmpfile")"
+
+     # Without a key we only (re)fetched the response
+     [[ -z "$key" ]] && return 0
+
+     # The key must be present, otherwise the API may have changed
+     [[ "$response" == *"\"$key\""* ]] || { debugme echo "HSTS preloadlist key unrecognized: $key"; return 21; }
+
+     # String values are quoted in the JSON, booleans are not, so accept either form
+     [[ "$response" == *"\"$key\": \"$value\""* || "$response" == *"\"$key\": $value"* ]] && return 10
+     return 20
+}
+
+# Returns the value of a known key from the hstspreload.org preload list API.
+# Depends on check_hsts_preloadlist_match().
+# arg1: domain to check
+# arg2: key to resolve (status or bulk)
+# Echoes the matched value and returns 0, or returns 1 if no known value matched.
+#
+check_hsts_preloadlist_value() {
+     local domain="$1"
+     local key="$2"
+     local -a values=()
+     local value
+     local value_ret=""
+
+     [[ -z "$key" ]] && return 1
+
+     # Only test against known values instead of echoing the API response back,
+     # so no untrusted input is reflected.
+     case "$key" in
+          status) values=("unknown" "pending" "rejected" "preloaded") ;;
+          bulk)   values=("true" "false") ;;
+          *)      return 1 ;;
+     esac
+
+     for value in "${values[@]}"; do
+          check_hsts_preloadlist_match "$domain" "$key" "$value"
+          [[ $? -eq 10 ]] && value_ret="$value" && break
+     done
+
+     [[ -n "$value_ret" ]] && safe_echo "$value_ret" && return 0
+     return 1
+}
+
 # waits maxsleep 1/10 seconds (arg2) until process with arg1 (pid) will be killed
 #
 # return values
@@ -2918,6 +2988,8 @@ run_hsts() {
      local hsts_age_days
      local spaces="                              "
      local jsonID="HSTS"
+     local json_postfix=""
+     local preloadmarked preloadsame preloadbulk preloadcombined=""
 
      if [[ ! -s $HEADERFILE ]]; then
           run_http_header "$1" || return 1
@@ -2971,18 +3043,105 @@ run_hsts() {
           fi
           if preload "$TMPFILE"; then
                fileout "${jsonID}_preload" "OK" "domain IS marked for preloading"
+               preloadmarked=true
           else
                fileout "${jsonID}_preload" "INFO" "domain is NOT marked for preloading"
-               #FIXME: To be checked against preloading lists,
-               # e.g. https://dxr.mozilla.org/mozilla-central/source/security/manager/boot/src/nsSTSPreloadList.inc
-               #      https://chromium.googlesource.com/chromium/src/+/master/net/http/transport_security_state_static.json
+               preloadmarked=false
           fi
      else
           pr_svrty_low "not offered"
           fileout "$jsonID" "LOW" "not offered"
+          preloadmarked=false
      fi
      outln
 
+     # Check the domain against the hstspreload.org HSTS preload list (requires --phone-out).
+     # Run this regardless of the served header: a domain may still be listed after the header
+     # was removed, or be rejected because the served header does not meet the requirements.
+     if "$PHONE_OUT"; then
+          json_postfix="_preloadAPI"
+          pr_bold " HSTS preload API             "
+
+          # If the domain itself is the preloaded entry, it may be fine that the header omits 'preload'
+          check_hsts_preloadlist_match "$NODE" "preloadedDomain" "$NODE"
+          [[ $? -eq 10 ]] && preloadsame=true || preloadsame=false
+
+          # bulk=true: added via the submission form; false: manual addition or a subdomain
+          check_hsts_preloadlist_match "$NODE" "bulk" "true"
+          [[ $? -eq 10 ]] && preloadbulk=true || preloadbulk=false
+
+          # Combine the three booleans for a compact lookup, e.g. marked+same+bulk -> "111"
+          [[ $preloadmarked == true ]] && preloadcombined="${preloadcombined}1" || preloadcombined="${preloadcombined}0"
+          [[ $preloadsame == true ]] && preloadcombined="${preloadcombined}1" || preloadcombined="${preloadcombined}0"
+          [[ $preloadbulk == true ]] && preloadcombined="${preloadcombined}1" || preloadcombined="${preloadcombined}0"
+          debugme echo "Temporary lookupvariable: $preloadcombined"
+
+          # Determine and show the outcome
+          case "$(check_hsts_preloadlist_value "$NODE" "status")" in
+               "unknown") # Not found in the HSTS preload list
+                    case "$preloadcombined" in
+                         "000"|"001"|"010"|"011")
+                              outln "no entry"
+                              fileout "${jsonID}${json_postfix}" "INFO" "no entry"
+                              ;;
+                         "100"|"101"|"110"|"111")
+                              pr_svrty_low "no entry"
+                              outln " -- submit to HSTS preload list"; fileout "${jsonID}${json_postfix}" "LOW" "no entry"
+                              ;;
+                    esac
+                    ;;
+               "pending") # Currently in the HSTS pending list
+                    case "$preloadcombined" in
+                         "000"|"001"|"010"|"100"|"101"|"110"|"111")
+                              outln "pending"
+                              fileout "${jsonID}${json_postfix}" "INFO" "pending"
+                              ;;
+                         "011") pr_svrty_medium "pending"
+                              outln " -- addition going to fail, add header"
+                              fileout "${jsonID}${json_postfix}" "MEDIUM" "pending"
+                              ;;
+                    esac
+                    ;;
+               "rejected") # Entry is considered rejected by the HSTS list
+                    case "$preloadcombined" in
+                         "000"|"001"|"010"|"011")
+                              outln "rejected"
+                              fileout "${jsonID}${json_postfix}" "INFO" "rejected"
+                              ;;
+                         "100"|"101"|"110"|"111")
+                              pr_svrty_medium "rejected" ; outln " -- check other requirements"
+                              fileout "${jsonID}${json_postfix}" "MEDIUM" "rejected"
+                              ;;
+                    esac
+                    ;;
+               "preloaded") # Marked as 'preload' in the HSTS preload list
+                    case "$preloadcombined" in
+                         "000"|"001")
+                              prln_svrty_good "preloaded"
+                              fileout "${jsonID}${json_postfix}" "OK" "preloaded"
+                              ;;
+                         "010")
+                              outln "preloaded -- manual addition detected"
+                              fileout "${jsonID}${json_postfix}" "INFO" "preloaded"
+                              ;;
+                         "011")
+                              pr_svrty_medium "preloaded"
+                              outln " -- list may remove entry, add header"
+                              fileout "${jsonID}${json_postfix}" "MEDIUM" "preloaded"
+                              ;;
+                         "100"|"101"|"110"|"111")
+                              prln_svrty_best "preloaded"
+                              fileout "${jsonID}${json_postfix}" "OK" "preloaded"
+                              ;;
+                    esac
+                    ;;
+               *) # Empty: the hstspreload.org API was unreachable or returned an unexpected response
+                    prln_warning "not checked (HSTS preload list lookup failed)"
+                    fileout "${jsonID}${json_postfix}" "WARN" "HSTS preload list could not be checked"
+                    ;;
+          esac
+     fi
+
      tmpfile_handle ${FUNCNAME[0]}.txt
      return 0
 }
@@ -21708,7 +21867,7 @@ tuning / connect options (most also can be preset via environment variables):
      --sneaky                      leave less traces in target logs: user agent, referer
      --user-agent <user agent>     set a custom user agent instead of the standard user agent
      --ids-friendly                skips a few vulnerability checks which may cause IDSs to block the scanning IP
-     --phone-out                   allow to contact external servers for CRL download and querying OCSP responder
+     --phone-out                   allow to contact external servers for CRL download, querying OCSP responder and the HSTS preload API
      --add-ca <CA files|CA dir>    path to <CAdir> with *.pem or a comma separated list of CA files to include in trust check
      --mtls <CLIENT CERT file>     path to <CLIENT CERT> file in PEM format containing unencrypted certificate key (beta)
      --basicauth <user:pass>       provide HTTP basic auth information