diff --git a/backend/src/routes/auth.ts b/backend/src/routes/auth.ts
index 7e77426..01b3474 100644
--- a/backend/src/routes/auth.ts
+++ b/backend/src/routes/auth.ts
@@ -1,6 +1,6 @@
 import { randomBytes } from "node:crypto";
 import argon2 from "argon2";
-import { eq } from "drizzle-orm";
+import { eq, sql } from "drizzle-orm";
 import type { FastifyInstance } from "fastify";
 import { z } from "zod";
 import { db } from "../db/client.js";
@@ -129,7 +129,7 @@ export async function authRoutes(app: FastifyInstance) {
 			const { username, password } = parsed.data;
 
 			// Check if username already exists
-			const [existingUser] = await db.select().from(users).where(eq(users.username, username));
+			const [existingUser] = await db.select().from(users).where(sql`lower(${users.username}) = lower(${username})`);
 			if (existingUser) {
 				return reply.status(409).send({ error: "Username already taken", code: "USERNAME_EXISTS" });
 			}
@@ -190,7 +190,7 @@ export async function authRoutes(app: FastifyInstance) {
 			const { username, password, rememberMe } = parsed.data;
 
 			// Find user by username
-			const [user] = await db.select().from(users).where(eq(users.username, username));
+			const [user] = await db.select().from(users).where(sql`lower(${users.username}) = lower(${username})`);
 
 			// Generic error to prevent user enumeration
 			const invalidCredentialsError = () =>
diff --git a/backend/src/routes/oidc.ts b/backend/src/routes/oidc.ts
index a9cc089..555f4dc 100644
--- a/backend/src/routes/oidc.ts
+++ b/backend/src/routes/oidc.ts
@@ -1,5 +1,5 @@
 import { createHash, randomBytes } from "node:crypto";
-import { eq } from "drizzle-orm";
+import { eq, sql } from "drizzle-orm";
 import type { FastifyInstance, FastifyReply } from "fastify";
 import * as client from "openid-client";
 import { db } from "../db/client.js";
@@ -234,7 +234,7 @@ async function findOrCreateOIDCUser(
 	}
 
 	// Check if username already exists (potential collision)
-	const [existingByUsername] = await db.select().from(users).where(eq(users.username, username));
+	const [existingByUsername] = await db.select().from(users).where(sql`lower(${users.username}) = lower(${username})`);
 
 	if (existingByUsername) {
 		// Username collision! Check if it's a local user without OIDC linked
diff --git a/backend/src/test/auth.test.ts b/backend/src/test/auth.test.ts
index eb1abeb..d51b6c9 100644
--- a/backend/src/test/auth.test.ts
+++ b/backend/src/test/auth.test.ts
@@ -194,6 +194,29 @@ describe("Auth Routes (AUTH_ENABLED=true)", () => {
 			expect(response.json().code).toBe("USERNAME_EXISTS");
 		});
 
+		it("should reject duplicate username regardless of case", async () => {
+			await app.inject({
+				method: "POST",
+				url: "/auth/register",
+				payload: {
+					username: "CaseUser",
+					password: "TestPassword123",
+				},
+			});
+
+			const response = await app.inject({
+				method: "POST",
+				url: "/auth/register",
+				payload: {
+					username: "caseuser",
+					password: "AnotherPassword123",
+				},
+			});
+
+			expect(response.statusCode).toBe(409);
+			expect(response.json().code).toBe("USERNAME_EXISTS");
+		});
+
 		it("should reject short password", async () => {
 			const response = await app.inject({
 				method: "POST",
@@ -275,6 +298,21 @@ describe("Auth Routes (AUTH_ENABLED=true)", () => {
 			expect(cookies.find((c: any) => c.name === "refresh_token")).toBeDefined();
 		});
 
+		it("should login case-insensitively with different username casing", async () => {
+			const response = await app.inject({
+				method: "POST",
+				url: "/auth/login",
+				payload: {
+					username: "LOGINUSER",
+					password: "TestPassword123",
+				},
+			});
+
+			expect(response.statusCode).toBe(200);
+			expect(response.json().ok).toBe(true);
+			expect(response.json().user.username).toBe("loginuser");
+		});
+
 		it("should reject invalid password", async () => {
 			const response = await app.inject({
 				method: "POST",