diff --git a/packages/backend/src/server/api/endpoints/notes/create.test.ts b/packages/backend/src/server/api/endpoints/notes/create.test.ts
index 0aa05a2c93..3228bbd014 100644
--- a/packages/backend/src/server/api/endpoints/notes/create.test.ts
+++ b/packages/backend/src/server/api/endpoints/notes/create.test.ts
@@ -48,6 +48,11 @@ describe('api:notes/create', () => {
 				expect(v({ text: await tooLong }))
 					.toBe(INVALID);
 			});
+
+			test('whitespace-only post', () => {
+				expect(v({ text: ' ' }))
+					.toBe(INVALID);
+			});
 		});
 
 		describe('cw', () => {
diff --git a/packages/backend/src/server/api/endpoints/notes/create.ts b/packages/backend/src/server/api/endpoints/notes/create.ts
index 52a32b7fec..ed522e03c9 100644
--- a/packages/backend/src/server/api/endpoints/notes/create.ts
+++ b/packages/backend/src/server/api/endpoints/notes/create.ts
@@ -138,6 +138,7 @@ export const paramDef = {
 			minLength: 1,
 			maxLength: MAX_NOTE_TEXT_LENGTH,
 			nullable: true,
+			pattern: '[^\\s]+',
 		},
 		fileIds: {
 			type: 'array',
@@ -195,6 +196,7 @@ export const paramDef = {
 				minLength: 1,
 				maxLength: MAX_NOTE_TEXT_LENGTH,
 				nullable: false,
+				pattern: '[^\\s]+',
 			},
 		},
 		required: ['text'],