diff --git a/lib/Controller/BanController.php b/lib/Controller/BanController.php
index 43cf0bca13..0ed29b5ec4 100644
--- a/lib/Controller/BanController.php
+++ b/lib/Controller/BanController.php
@@ -41,7 +41,7 @@ class BanController extends AEnvironmentAwareController {
 	 * @param 'users'|'groups'|'circles'|'emails'|'federated_users'|'phones'|'ip' $actorType Type of actor to ban, or `ip` when banning a clients remote address
 	 * @psalm-param Attendee::ACTOR_*|'ip' $actorType Type of actor to ban, or `ip` when banning a clients remote address
 	 * @param string $actorId Actor ID or the IP address or range in case of type `ip`
-	 * @param string $internalNote Optional internal note
+	 * @param string $internalNote Optional internal note (max. 4000 characters)
 	 * @return DataResponse<Http::STATUS_OK, TalkBan, array{}>|DataResponse<Http::STATUS_BAD_REQUEST, array{error: 'bannedActor'|'internalNote'}, array{}>
 	 *
 	 * 200: Ban successfully
diff --git a/lib/Model/Ban.php b/lib/Model/Ban.php
index b2f1d686d4..52785731aa 100644
--- a/lib/Model/Ban.php
+++ b/lib/Model/Ban.php
@@ -32,6 +32,8 @@ use OCP\AppFramework\Db\Entity;
  * @method null|string getInternalNote()
  */
 class Ban extends Entity implements \JsonSerializable {
+	public const NOTE_MAX_LENGTH = 4000;
+
 	protected string $actorType = '';
 	protected string $actorId = '';
 	protected int $roomId = 0;
diff --git a/lib/Service/BanService.php b/lib/Service/BanService.php
index 5d8495db3b..0be4edf765 100644
--- a/lib/Service/BanService.php
+++ b/lib/Service/BanService.php
@@ -33,7 +33,7 @@ class BanService {
 			throw new \InvalidArgumentException('bannedActor');
 		}
 
-		if (empty($internalNote)) {
+		if (strlen($internalNote) > Ban::NOTE_MAX_LENGTH) {
 			throw new \InvalidArgumentException('internalNote');
 		}
 
diff --git a/openapi-full.json b/openapi-full.json
index f9f21adc89..32f12e5576 100644
--- a/openapi-full.json
+++ b/openapi-full.json
@@ -2003,7 +2003,7 @@
                     {
                         "name": "internalNote",
                         "in": "query",
-                        "description": "Optional internal note",
+                        "description": "Optional internal note (max. 4000 characters)",
                         "schema": {
                             "type": "string",
                             "default": ""
diff --git a/openapi.json b/openapi.json
index d70cb5769d..1ec339fbfd 100644
--- a/openapi.json
+++ b/openapi.json
@@ -1890,7 +1890,7 @@
                     {
                         "name": "internalNote",
                         "in": "query",
-                        "description": "Optional internal note",
+                        "description": "Optional internal note (max. 4000 characters)",
                         "schema": {
                             "type": "string",
                             "default": ""
diff --git a/src/types/openapi/openapi-full.ts b/src/types/openapi/openapi-full.ts
index 43e07580a2..291fe88ee8 100644
--- a/src/types/openapi/openapi-full.ts
+++ b/src/types/openapi/openapi-full.ts
@@ -2389,7 +2389,7 @@ export interface operations {
                 actorType: "users" | "groups" | "circles" | "emails" | "federated_users" | "phones" | "ip";
                 /** @description Actor ID or the IP address or range in case of type `ip` */
                 actorId: string;
-                /** @description Optional internal note */
+                /** @description Optional internal note (max. 4000 characters) */
                 internalNote?: string;
             };
             header: {
diff --git a/src/types/openapi/openapi.ts b/src/types/openapi/openapi.ts
index 1487919d2c..64cb4fd387 100644
--- a/src/types/openapi/openapi.ts
+++ b/src/types/openapi/openapi.ts
@@ -1874,7 +1874,7 @@ export interface operations {
                 actorType: "users" | "groups" | "circles" | "emails" | "federated_users" | "phones" | "ip";
                 /** @description Actor ID or the IP address or range in case of type `ip` */
                 actorId: string;
-                /** @description Optional internal note */
+                /** @description Optional internal note (max. 4000 characters) */
                 internalNote?: string;
             };
             header: {