diff --git a/build-tools-internal/src/integTest/groovy/org/elasticsearch/gradle/internal/transport/TransportVersionValidationFuncTest.groovy b/build-tools-internal/src/integTest/groovy/org/elasticsearch/gradle/internal/transport/TransportVersionValidationFuncTest.groovy
index 3778ea3e62914..36be4c4d94bff 100644
--- a/build-tools-internal/src/integTest/groovy/org/elasticsearch/gradle/internal/transport/TransportVersionValidationFuncTest.groovy
+++ b/build-tools-internal/src/integTest/groovy/org/elasticsearch/gradle/internal/transport/TransportVersionValidationFuncTest.groovy
@@ -223,10 +223,33 @@ class TransportVersionValidationFuncTest extends AbstractTransportVersionFuncTes
 
     def "unreferable definitions can have primary ids that are patches"() {
         given:
-        unreferableTransportVersion("initial_10.0.1", "10000001")
+        unreferableTransportVersion("initial_7.0.1", "7000001")
         when:
         def result = gradleRunner(":myserver:validateTransportVersionResources").build()
         then:
         result.task(":myserver:validateTransportVersionResources").outcome == TaskOutcome.SUCCESS
     }
+
+    def "highest id in an referable definition should exist in an upper bounds file"() {
+        given:
+        referableAndReferencedTransportVersion("some_tv", "10000000")
+        when:
+        def result = validateResourcesFails()
+        then:
+        assertValidateResourcesFailure(result, "Transport version definition file " +
+            "[myserver/src/main/resources/transport/definitions/referable/some_tv.csv] " +
+            "has the highest transport version id [10000000] but is not present in any upper bounds files")
+    }
+
+    def "highest id in an unreferable definition should exist in an upper bounds file"() {
+        given:
+        unreferableTransportVersion("initial_10.0.0", "10000000")
+        when:
+        def result = validateResourcesFails()
+        then:
+        // TODO: this should be _unreferable_ in the error message, but will require some rework
+        assertValidateResourcesFailure(result, "Transport version definition file " +
+            "[myserver/src/main/resources/transport/definitions/referable/initial_10.0.0.csv] " +
+            "has the highest transport version id [10000000] but is not present in any upper bounds files")
+    }
 }
diff --git a/build-tools-internal/src/main/java/org/elasticsearch/gradle/internal/transport/ValidateTransportVersionResourcesTask.java b/build-tools-internal/src/main/java/org/elasticsearch/gradle/internal/transport/ValidateTransportVersionResourcesTask.java
index 44e9bde8352f2..222683d4514e5 100644
--- a/build-tools-internal/src/main/java/org/elasticsearch/gradle/internal/transport/ValidateTransportVersionResourcesTask.java
+++ b/build-tools-internal/src/main/java/org/elasticsearch/gradle/internal/transport/ValidateTransportVersionResourcesTask.java
@@ -83,6 +83,8 @@ public void validateTransportVersions() throws IOException {
         for (var upperBound : upperBounds.values()) {
             validateUpperBound(upperBound, allDefinitions, idsByBase);
         }
+
+        validateLargestIdIsUsed(upperBounds, allDefinitions);
     }
 
     private Map<String, TransportVersionDefinition> collectAllDefinitions(
@@ -253,6 +255,27 @@ private void validateBase(int base, List<IdAndDefinition> ids) {
         }
     }
 
+    private void validateLargestIdIsUsed(
+        Map<String, TransportVersionUpperBound> upperBounds,
+        Map<String, TransportVersionDefinition> allDefinitions
+    ) {
+        // first id is always the highest within a definition, and validated earlier
+        // note we use min instead of max because the id comparator is in descending order
+        var highestDefinition = allDefinitions.values().stream().min(Comparator.comparing(d -> d.ids().get(0))).get();
+        var highestId = highestDefinition.ids().get(0);
+
+        for (var upperBound : upperBounds.values()) {
+            if (upperBound.id().equals(highestId)) {
+                return;
+            }
+        }
+
+        throwDefinitionFailure(
+            highestDefinition,
+            "has the highest transport version id [" + highestId + "] but is not present in any upper bounds files"
+        );
+    }
+
     private void throwDefinitionFailure(TransportVersionDefinition definition, String message) {
         Path relativePath = getResources().get().getReferableDefinitionRepositoryPath(definition);
         throw new IllegalStateException("Transport version definition file [" + relativePath + "] " + message);