diff --git a/backend/src/validators/user.validator.ts b/backend/src/validators/user.validator.ts
index ce9a6c30..45bd5521 100644
--- a/backend/src/validators/user.validator.ts
+++ b/backend/src/validators/user.validator.ts
@@ -1,7 +1,31 @@
 import { z } from 'zod';
 
+/**
+ * Stellar Ed25519 account IDs are base32 strings: a `G` version byte prefix
+ * followed by 55 characters from the RFC 4648 base32 alphabet, 56 in total.
+ *
+ * This is a format check only — it deliberately does not verify the trailing
+ * CRC16 checksum, so callers must not treat a match as proof the key exists
+ * or was typed correctly.
+ */
+export const STELLAR_PUBLIC_KEY_REGEX = /^G[A-Z2-7]{55}$/;
+
+export const STELLAR_PUBLIC_KEY_ERROR =
+    'Invalid Stellar public key format: expected 56 base32 characters starting with "G"';
+
+/**
+ * Reusable schema for a Stellar account public key.
+ *
+ * Rejects malformed keys at the validation layer so they never reach the
+ * controller or repository, where they would surface as opaque lookup misses
+ * or be stored as-is.
+ */
+export const stellarPublicKeySchema = z
+    .string({ message: 'publicKey is required and must be a string' })
+    .regex(STELLAR_PUBLIC_KEY_REGEX, STELLAR_PUBLIC_KEY_ERROR);
+
 export const registerUserSchema = z.object({
-    publicKey: z.string().min(50, 'Invalid Stellar public key').regex(/^G[A-Z2-7]{55}$/, 'Invalid Stellar public key format'),
+    publicKey: stellarPublicKeySchema,
 });
 
 export type RegisterUserInput = z.infer<typeof registerUserSchema>;
diff --git a/backend/tests/user.validator.test.ts b/backend/tests/user.validator.test.ts
new file mode 100644
index 00000000..9df67687
--- /dev/null
+++ b/backend/tests/user.validator.test.ts
@@ -0,0 +1,68 @@
+import { describe, it, expect } from 'vitest';
+import { ZodError } from 'zod';
+import {
+  registerUserSchema,
+  STELLAR_PUBLIC_KEY_ERROR,
+} from '../src/validators/user.validator.js';
+
+const VALID_KEY = 'GD2XP6FNWL6IWULVMPNA2RV2T7GLCJHK3RH75GBCY7TSVIWDITJN4FXJ';
+
+describe('User Validator', () => {
+  it('should accept a well-formed Stellar public key', () => {
+    const result = registerUserSchema.safeParse({ publicKey: VALID_KEY });
+    expect(result.success).toBe(true);
+  });
+
+  it('should reject a malformed public key with a descriptive message', () => {
+    const result = registerUserSchema.safeParse({ publicKey: 'not-a-stellar-key' });
+
+    expect(result.success).toBe(false);
+    expect(result.error?.issues).toHaveLength(1);
+    expect(result.error?.issues[0]?.path).toEqual(['publicKey']);
+    expect(result.error?.issues[0]?.message).toBe(STELLAR_PUBLIC_KEY_ERROR);
+  });
+
+  it.each([
+    ['too short', 'GD2XP6FNWL6IWULV'],
+    ['too long', `${VALID_KEY}AAAA`],
+    ['using a wrong version prefix', `S${VALID_KEY.slice(1)}`],
+    ['lowercase', VALID_KEY.toLowerCase()],
+    ['outside the base32 alphabet', `${VALID_KEY.slice(0, 55)}1`],
+    ['an empty string', ''],
+    ['whitespace padded', ` ${VALID_KEY} `],
+  ])('should reject a key that is %s', (_label, publicKey) => {
+    const result = registerUserSchema.safeParse({ publicKey });
+
+    expect(result.success).toBe(false);
+    expect(result.error?.issues[0]?.message).toBe(STELLAR_PUBLIC_KEY_ERROR);
+  });
+
+  it.each([
+    ['missing', undefined],
+    ['a number', 12345],
+    ['null', null],
+  ])('should reject a publicKey that is %s', (_label, publicKey) => {
+    const result = registerUserSchema.safeParse({ publicKey });
+
+    expect(result.success).toBe(false);
+    expect(result.error?.issues[0]?.message).toBe(
+      'publicKey is required and must be a string',
+    );
+  });
+
+  // `registerUser` calls `.parse`, so a malformed key throws before any Prisma
+  // access. The global error middleware turns that ZodError into a 400 carrying
+  // the message asserted above (see tests/error.middleware.test.ts).
+  it('should throw a ZodError so the request fails before the controller body runs', () => {
+    const parse = () => registerUserSchema.parse({ publicKey: 'not-a-stellar-key' });
+
+    expect(parse).toThrow(ZodError);
+
+    try {
+      parse();
+      expect.unreachable('parse should have thrown');
+    } catch (error) {
+      expect((error as ZodError).issues[0]?.message).toBe(STELLAR_PUBLIC_KEY_ERROR);
+    }
+  });
+});