diff --git a/backend/src/predictions/dto/update-prediction-note.dto.ts b/backend/src/predictions/dto/update-prediction-note.dto.ts
index 7467b092..2077edaa 100644
--- a/backend/src/predictions/dto/update-prediction-note.dto.ts
+++ b/backend/src/predictions/dto/update-prediction-note.dto.ts
@@ -1,6 +1,37 @@
 import { IsString, MaxLength } from 'class-validator';
 import { ApiProperty } from '@nestjs/swagger';
 
+/**
+ * Strip HTML markup from a user-supplied prediction note.
+ *
+ * Notes are personal free text and are never rendered as markup, so tags carry
+ * no meaning here and are removed rather than escaped. Removing them keeps the
+ * stored value equal to what the user meant to write, and leaves nothing for a
+ * downstream consumer to mis-render.
+ *
+ * Applied in the service rather than as a `@Transform` decorator: the note
+ * route does not enable `transform: true` on its ValidationPipe, so a decorator
+ * would silently not run. Sanitising at the write keeps it independent of pipe
+ * configuration.
+ *
+ * @param input - The user-provided note
+ * @returns The note with markup removed and surrounding whitespace trimmed
+ *
+ * @example
+ * sanitizeNote('<script>alert(1)</script>hi') // returns 'hi'
+ * sanitizeNote('  spaced  ')                  // returns 'spaced'
+ */
+export function sanitizeNote(input: string): string {
+  if (!input) return input;
+  return input
+    // Drop script and style bodies wholesale. Removing only the tags would
+    // leave the code itself sitting in the note as plain text.
+    .replace(/<(script|style)\b[^>]*>[\s\S]*?<\/\1>/gi, '')
+    // Remaining tags, including unclosed ones such as `<img src=x onerror=...`.
+    .replace(/<[^>]*>?/g, '')
+    .trim();
+}
+
 export class UpdatePredictionNoteDto {
   @ApiProperty({
     description: 'Personal note for the prediction',
diff --git a/backend/src/predictions/predictions.service.spec.ts b/backend/src/predictions/predictions.service.spec.ts
index 6ef08512..e94bcfb3 100644
--- a/backend/src/predictions/predictions.service.spec.ts
+++ b/backend/src/predictions/predictions.service.spec.ts
@@ -861,6 +861,34 @@ describe('PredictionsService', () => {
       ).rejects.toThrow(PredictionNotFoundException);
       expect(submitPrediction).not.toHaveBeenCalled();
     });
+
+    it.each([
+      ['<script>alert(1)</script>Real analysis', 'Real analysis'],
+      ['<img src=x onerror=alert(1)>note', 'note'],
+      ['<b>bold</b> and <i>italic</i>', 'bold and italic'],
+      ['<style>body{}</style>clean', 'clean'],
+      ['  padded  ', 'padded'],
+      ['plain text, unchanged', 'plain text, unchanged'],
+    ])('should sanitize %j before saving', async (input, expected) => {
+      const user = makeUser();
+      const prediction = {
+        id: 'pred-1',
+        user,
+        market: makeMarket(),
+        note: null,
+      } as unknown as Prediction;
+
+      mockPredictionsRepo.findOne.mockResolvedValue(prediction);
+      mockPredictionsRepo.save.mockImplementation(async (p) => p as Prediction);
+
+      await service.updateNote('pred-1', { note: input }, user);
+
+      // Assert on what reaches the repository, not on the return value: the
+      // sanitised text is what actually gets stored.
+      expect(mockPredictionsRepo.save).toHaveBeenCalledWith(
+        expect.objectContaining({ note: expected }),
+      );
+    });
   });
 
   describe('findById', () => {
diff --git a/backend/src/predictions/predictions.service.ts b/backend/src/predictions/predictions.service.ts
index 967bbf94..65b73d91 100644
--- a/backend/src/predictions/predictions.service.ts
+++ b/backend/src/predictions/predictions.service.ts
@@ -22,7 +22,10 @@ import {
   BatchPredictionResultDto,
   BatchSubmitResponseDto,
 } from './dto/batch-submit-response.dto';
-import { UpdatePredictionNoteDto } from './dto/update-prediction-note.dto';
+import {
+  UpdatePredictionNoteDto,
+  sanitizeNote,
+} from './dto/update-prediction-note.dto';
 import {
   ListMarketPredictionsDto,
   MarketPredictionResponseDto,
@@ -712,7 +715,7 @@ export class PredictionsService {
       throw new PredictionNotFoundException(predictionId);
     }
 
-    prediction.note = dto.note;
+    prediction.note = sanitizeNote(dto.note);
     return this.predictionsRepository.save(prediction);
   }