diff --git a/src/services/FormsService.php b/src/services/FormsService.php
index b771d420..42e566ed 100755
--- a/src/services/FormsService.php
+++ b/src/services/FormsService.php
@@ -173,6 +173,7 @@ public function createAndSubscribeContact(string $email, array|null $fieldValues
         }
 
         $contact->setScenario(Element::SCENARIO_LIVE);
+        $source = $this->getTruncatedSource($source);
 
         // If subscribe verification required
         if ($mailingList->getMailingListType()->subscribeVerificationRequired) {
@@ -216,7 +217,7 @@ public function createAndSubscribeContact(string $email, array|null $fieldValues
     public function subscribeContact(ContactElement $contact, MailingListElement $mailingList, string $sourceType = null, string $source = null, bool $verified = null): void
     {
         $sourceType = $sourceType ?? '';
-        $source = $source ?? '';
+        $source = $this->getTruncatedSource($source);
         $verified = $verified ?? false;
 
         // Fire a before event
@@ -327,4 +328,18 @@ public function sendEmail(string $email, string $subject, string $htmlBody, stri
 
         return $message->send();
     }
+
+    /**
+     * Returns a truncated source if longer than the allowed length.
+     */
+    private function getTruncatedSource(?string $source): string
+    {
+        if ($source === null) {
+            return '';
+        }
+
+        $suffix = '...';
+
+        return StringHelper::truncate($source, 255 - strlen($suffix), $suffix);
+    }
 }
diff --git a/tests/TESTS.md b/tests/TESTS.md
index d43695ba..97a8dd16 100644
--- a/tests/TESTS.md
+++ b/tests/TESTS.md
@@ -34,6 +34,7 @@ _Tests interacting with contacts via forms._
 ![Pass](https://raw.githubusercontent.com/putyourlightson/craft-generate-test-spec/main/icons/pass.svg) A verify subscribe email is sent to a pending contact on subscribe.  
 ![Pass](https://raw.githubusercontent.com/putyourlightson/craft-generate-test-spec/main/icons/pass.svg) A verify unsubscribe email is sent to a contact on unsubscribe.  
 ![Pass](https://raw.githubusercontent.com/putyourlightson/craft-generate-test-spec/main/icons/pass.svg) Subscribing and then unsubscribing a contact works.  
+![Pass](https://raw.githubusercontent.com/putyourlightson/craft-generate-test-spec/main/icons/pass.svg) Subscribing results in a truncated source if too long.  
 ![Pass](https://raw.githubusercontent.com/putyourlightson/craft-generate-test-spec/main/icons/pass.svg) Updating a contact modifies its last activity timestamp.  
 
 ### [Import](pest/Feature/ImportTest.php)
diff --git a/tests/pest/Feature/FormTest.php b/tests/pest/Feature/FormTest.php
index 4d85233b..5096178d 100644
--- a/tests/pest/Feature/FormTest.php
+++ b/tests/pest/Feature/FormTest.php
@@ -1,7 +1,9 @@
 <?php
 
+use craft\helpers\StringHelper;
 use putyourlightson\campaign\Campaign;
 use putyourlightson\campaign\models\PendingContactModel;
+use putyourlightson\campaign\records\ContactMailingListRecord;
 use putyourlightson\campaign\services\FormsService;
 
 /**
@@ -49,7 +51,25 @@
     $subscribedMailingLists = $contact->getSubscribedMailingLists();
 
     expect($subscribedMailingLists)
-        ->toHaveCount(0);
+        ->toBeEmpty();
+});
+
+test('Subscribing results in a truncated source if too long', function() {
+    $mailingList = createMailingList();
+    $contact = createContact();
+    $source = StringHelper::randomString(252);
+    Campaign::$plugin->forms->subscribeContact($contact, $mailingList, 'web', $source . StringHelper::randomString(10));
+
+    /** @var ContactMailingListRecord $contactMailingListRecord */
+    $contactMailingListRecord = ContactMailingListRecord::find()
+        ->where([
+            'contactId' => $contact->id,
+            'mailingListId' => $mailingList->id,
+        ])
+        ->one();
+
+    expect($contactMailingListRecord->source)
+        ->toBe($source . '...');
 });
 
 test('Updating a contact modifies its last activity timestamp', function() {