diff --git a/server/package-lock.json b/server/package-lock.json
index e97d1f1..3f79208 100644
--- a/server/package-lock.json
+++ b/server/package-lock.json
@@ -27,7 +27,8 @@
         "passport-google-oauth20": "^2.0.0",
         "qs": "6.14.1",
         "redis": "^5.8.3",
-        "socket.io": "^4.8.1"
+        "socket.io": "^4.8.1",
+        "zod": "^4.3.5"
       },
       "devDependencies": {
         "@flydotio/dockerfile": "^0.7.10",
@@ -3105,6 +3106,7 @@
       "version": "2.3.3",
       "resolved": "https://registry.npmjs.org/fsevents/-/fsevents-2.3.3.tgz",
       "integrity": "sha512-5xoDfX+fL7faATnagmWPpbFtwh/R77WmMMqqHGS65C3vvB0YHrgF+B1YmZ3441tMj5n63k0212XNoJwzlhffQw==",
+      "dev": true,
       "hasInstallScript": true,
       "license": "MIT",
       "optional": true,
@@ -4582,6 +4584,15 @@
       "funding": {
         "url": "https://github.com/sponsors/sindresorhus"
       }
+    },
+    "node_modules/zod": {
+      "version": "4.3.5",
+      "resolved": "https://registry.npmjs.org/zod/-/zod-4.3.5.tgz",
+      "integrity": "sha512-k7Nwx6vuWx1IJ9Bjuf4Zt1PEllcwe7cls3VNzm4CQ1/hgtFUK2bRNG3rvnpPUhFjmqJKAKtjV576KnUkHocg/g==",
+      "license": "MIT",
+      "funding": {
+        "url": "https://github.com/sponsors/colinhacks"
+      }
     }
   }
 }
diff --git a/server/package.json b/server/package.json
index 47df894..08a0269 100644
--- a/server/package.json
+++ b/server/package.json
@@ -31,7 +31,8 @@
     "passport-google-oauth20": "^2.0.0",
     "qs": "6.14.1",
     "redis": "^5.8.3",
-    "socket.io": "^4.8.1"
+    "socket.io": "^4.8.1",
+    "zod": "^4.3.5"
   },
   "devDependencies": {
     "@flydotio/dockerfile": "^0.7.10",
diff --git a/server/routes/auth.js b/server/routes/auth.js
index 6eb2a55..445734d 100644
--- a/server/routes/auth.js
+++ b/server/routes/auth.js
@@ -3,8 +3,10 @@ import bcrypt from "bcrypt";
 import jwt from "jsonwebtoken";
 import prisma from "../db.js";
 import { generateOTP, sendOTPEmail, isOTPEnabled } from "../utils/email.js";
+import { signupSchema, resetPasswordSchema } from "../schemas/auth.schema. js";
 import passport from "../config/passport.js";
 
+
 const router = express.Router();
 
 // Generate JWT token
@@ -141,23 +143,14 @@ router.post("/signup", async (req, res) => {
 
     console.log("🔐 Signup attempt:", { username, email });
 
-    if (!username || !email || !password) {
-      return res
-        .status(400)
-        .json({ error: "Username, email, and password are required" });
-    }
-
-    if (username.length < 3) {
-      return res
-        .status(400)
-        .json({ error: "Username must be at least 3 characters" });
-    }
-
-    if (password.length < 6) {
-      return res
-        .status(400)
-        .json({ error: "Password must be at least 6 characters long" });
-    }
+    // Validate request body with Zod schema
+try {
+  signupSchema.parse({ username, email, password, otp });
+} catch (zodError) {
+  // Format Zod errors into user-friendly messages
+  const errors = zodError.errors.map(err => err.message).join('; ');
+  return res.status(400).json({ error: errors });
+}
 
     // Verify OTP if enabled
     if (isOTPEnabled()) {
@@ -342,17 +335,14 @@ router.post("/reset-password", async (req, res) => {
   try {
     const { email, otp, newPassword } = req.body;
 
-    if (!email || !otp || !newPassword) {
-      return res
-        .status(400)
-        .json({ error: "Email, OTP, and new password are required" });
-    }
-
-    if (newPassword.length < 6) {
-      return res
-        .status(400)
-        .json({ error: "Password must be at least 6 characters long" });
-    }
+    // Validate request body with Zod schema
+try {
+  resetPasswordSchema.parse({ email, otp, newPassword });
+} catch (zodError) {
+  // Format Zod errors into user-friendly messages
+  const errors = zodError.errors.map(err => err.message).join('; ');
+  return res.status(400).json({ error: errors });
+}
 
     // Verify OTP
     const otpRecord = await prisma.otp.findFirst({
diff --git a/server/schemas/auth.schema.js b/server/schemas/auth.schema.js
new file mode 100644
index 0000000..cd93e64
--- /dev/null
+++ b/server/schemas/auth.schema.js
@@ -0,0 +1,85 @@
+import { z } from 'zod';
+
+/**
+ * Password validation schema using Zod
+ * Aligned with frontend PasswordInput component from PR #85
+ * 
+ * Criteria (matching frontend strength calculator):
+ * - Minimum 8 characters
+ * - At least one uppercase letter
+ * - At least one lowercase letter  
+ * - At least one number
+ * - At least one special character
+ */
+export const passwordSchema = z
+  .string()
+  .min(8, 'Password must be at least 8 characters long')
+  .regex(/[a-z]/, 'Password must include at least one lowercase letter')
+  .regex(/[A-Z]/, 'Password must include at least one uppercase letter')
+  .regex(/[0-9]/, 'Password must include at least one number')
+  .regex(
+    /[^A-Za-z0-9]/,
+    'Password must include at least one special character (! @#$%^&*... )'
+  )
+  .refine(
+    (password) => {
+      // Block common passwords
+      const commonPasswords = [
+        'password', 'password123', '123456', '123456789', '12345678',
+        'qwerty', 'abc123', 'monkey', '1234567', 'letmein',
+        'trustno1', 'dragon', 'baseball', 'iloveyou', 'master',
+        'sunshine', 'ashley', 'bailey', 'shadow', 'superman',
+        'qazwsx', 'welcome', 'admin', 'login', 'passw0rd'
+      ];
+      return !commonPasswords.includes(password.toLowerCase());
+    },
+    { message: 'This password is too common.  Please choose a more secure password' }
+  )
+  .refine(
+    (password) => {
+      // Block sequential characters (abc, 123)
+      const sequential = /(abc|bcd|cde|def|efg|fgh|ghi|hij|ijk|jkl|klm|lmn|mno|nop|opq|pqr|qrs|rst|stu|tuv|uvw|vwx|wxy|xyz|012|123|234|345|456|567|678|789)/i;
+      return !sequential.test(password);
+    },
+    { message: 'Password should not contain sequential characters (e.g., "abc", "123")' }
+  )
+  .refine(
+    (password) => {
+      // Block repeated characters (aaa, 111)
+      return !/(. )\1{2,}/.test(password);
+    },
+    { message: 'Password should not contain repeated characters (e.g., "aaa", "111")' }
+  );
+
+/**
+ * Signup request validation schema
+ */
+export const signupSchema = z. object({
+  username: z
+    .string()
+    .min(3, 'Username must be at least 3 characters')
+    .max(30, 'Username must not exceed 30 characters')
+    .regex(/^[a-zA-Z0-9_]+$/, 'Username can only contain letters, numbers, and underscores'),
+  email: z
+    . string()
+    .email('Please enter a valid email address'),
+  password: passwordSchema,
+  otp: z.string().optional()
+});
+
+/**
+ * Reset password request validation schema
+ */
+export const resetPasswordSchema = z. object({
+  email: z. string().email('Please enter a valid email address'),
+  otp: z.string().min(6, 'OTP must be 6 digits'),
+  newPassword: passwordSchema
+});
+
+/**
+ * Login request validation schema
+ */
+export const loginSchema = z.object({
+  usernameOrEmail: z.string().min(1, 'Username or email is required'),
+  password: z.string().min(1, 'Password is required')
+});
\ No newline at end of file