diff --git a/src/main/java/org/jenkinsci/plugins/workflow/support/steps/build/RunWrapper.java b/src/main/java/org/jenkinsci/plugins/workflow/support/steps/build/RunWrapper.java
index f59ed082..2214db77 100644
--- a/src/main/java/org/jenkinsci/plugins/workflow/support/steps/build/RunWrapper.java
+++ b/src/main/java/org/jenkinsci/plugins/workflow/support/steps/build/RunWrapper.java
@@ -314,6 +314,11 @@ public String getId() throws AbortException {
         return build().getId();
     }
 
+    @Whitelisted
+    public String getExternalizableId() {
+        return externalizableId;
+    }
+
     /**
      * Get environment variables defined in the build.
      *
diff --git a/src/main/resources/org/jenkinsci/plugins/workflow/support/steps/build/RunWrapper/help.html b/src/main/resources/org/jenkinsci/plugins/workflow/support/steps/build/RunWrapper/help.html
index 90ecf847..0c0a35ab 100644
--- a/src/main/resources/org/jenkinsci/plugins/workflow/support/steps/build/RunWrapper/help.html
+++ b/src/main/resources/org/jenkinsci/plugins/workflow/support/steps/build/RunWrapper/help.html
@@ -12,6 +12,7 @@
     <dt><code>fullProjectName</code></dt><dd>Full name of the project of this build, including folders such as <code>folder1/folder2/foo</code>.</dd>
     <dt><code>description</code></dt><dd>additional information about the build</dd>
     <dt><code>id</code></dt><dd>normally <code>number</code> as a string</dd>
+    <dt><code>externalizableId</code></dt><dd>identifier for this build from combining <code>fullProjectName</code> and <code>number</code> as <code>fullProjectName#number</code></dd>
     <dt><code>timeInMillis</code></dt><dd>time since the epoch when the build was scheduled</dd>
     <dt><code>startTimeInMillis</code></dt><dd>time since the epoch when the build started running</dd>
     <dt><code>duration</code></dt><dd>duration of the build in milliseconds</dd>
diff --git a/src/test/java/org/jenkinsci/plugins/workflow/support/steps/build/RunWrapperTest.java b/src/test/java/org/jenkinsci/plugins/workflow/support/steps/build/RunWrapperTest.java
index 859fc466..9defe7ff 100644
--- a/src/test/java/org/jenkinsci/plugins/workflow/support/steps/build/RunWrapperTest.java
+++ b/src/test/java/org/jenkinsci/plugins/workflow/support/steps/build/RunWrapperTest.java
@@ -287,6 +287,17 @@ public void upstreamBuilds() throws Throwable {
         });
     }
 
+    @Test
+    @Issue("JENKINS-73421")
+    public void externalizableId() throws Throwable {
+        sessions.then(j -> {
+            WorkflowJob first = j.createProject(WorkflowJob.class, "first-job");
+            first.setDefinition(new CpsFlowDefinition("echo(/externalizableId=$currentBuild.externalizableId/)", true));
+            WorkflowRun firstRun = j.buildAndAssertSuccess(first);
+            j.assertLogContains("externalizableId=" + firstRun.getExternalizableId(), firstRun);
+        });
+    }
+
     // Like org.hamcrest.text.MatchesPattern.matchesPattern(String) but doing a substring, not whole-string, match:
     private static Matcher<String> containsRegexp(final String rx) {
         return new SubstringMatcher("containing the regexp", false, rx) {