diff --git a/server/src/app.ts b/server/src/app.ts
index 7e7c1ce..4f2cb51 100644
--- a/server/src/app.ts
+++ b/server/src/app.ts
@@ -2,6 +2,7 @@ import { Hono } from "hono";
 import { getCookie } from "hono/cookie";
 import type { Db } from "./db";
 import { authRoutes, isAuthenticated } from "./routes/auth";
+import { settingsRoutes } from "./routes/settings";
 
 export type Deps = { db: Db; key: Buffer };
 const PUBLIC = new Set(["/api/health", "/api/me", "/api/setup", "/api/login"]);
@@ -17,7 +18,7 @@ export function createApp(deps: Deps) {
     return next();
   });
   app.route("/api", authRoutes({ db: deps.db }));
-  // Later route groups (settings, connectors, chat) mount here.
-  app.get("/api/settings", (c) => c.json({ error: "not implemented" }, 501)); // replaced in Task 5
+  app.route("/api", settingsRoutes(deps));
+  // Later route groups (connectors, chat) mount here.
   return app;
 }
diff --git a/server/src/routes/settings.ts b/server/src/routes/settings.ts
new file mode 100644
index 0000000..081abb7
--- /dev/null
+++ b/server/src/routes/settings.ts
@@ -0,0 +1,54 @@
+import { eq } from "drizzle-orm";
+import { Hono } from "hono";
+import { SettingsUpdate } from "@helios/shared";
+import type { Db } from "../db";
+import { settings } from "../db/schema";
+import { decrypt, encrypt, mask } from "../lib/crypto";
+
+const DEFAULTS = {
+  llm_base_url: "https://openrouter.ai/api/v1",
+  llm_model: "anthropic/claude-sonnet-4.5",
+} as const;
+
+async function get(db: Db, key: string): Promise<string | undefined> {
+  return (await db.select().from(settings).where(eq(settings.key, key))).at(0)?.value;
+}
+
+async function put(db: Db, key: string, value: string): Promise<void> {
+  await db.insert(settings).values({ key, value }).onConflictDoUpdate({ target: settings.key, set: { value } });
+}
+
+/** Decrypted read for other route groups (chat, connectors). */
+export async function getSetting(db: Db, key: string, cryptoKey: Buffer): Promise<string | undefined> {
+  const v = await get(db, key);
+  return v?.startsWith("enc:") ? decrypt(cryptoKey, v) : v;
+}
+
+export function settingsRoutes(deps: { db: Db; key: Buffer }) {
+  const app = new Hono();
+
+  app.get("/settings", async (c) => {
+    const [base, model, sealed] = await Promise.all([
+      get(deps.db, "llm_base_url"),
+      get(deps.db, "llm_model"),
+      get(deps.db, "llm_key"),
+    ]);
+    return c.json({
+      llmBaseUrl: base ?? DEFAULTS.llm_base_url,
+      llmModel: model ?? DEFAULTS.llm_model,
+      llmKeyMasked: sealed ? mask(decrypt(deps.key, sealed)) : null,
+    });
+  });
+
+  app.put("/settings", async (c) => {
+    const body = SettingsUpdate.safeParse(await c.req.json().catch(() => null));
+    if (!body.success) return c.json({ error: body.error.issues[0]?.message ?? "invalid body" }, 400);
+    const { llmBaseUrl, llmModel, llmKey } = body.data;
+    if (llmBaseUrl) await put(deps.db, "llm_base_url", llmBaseUrl);
+    if (llmModel) await put(deps.db, "llm_model", llmModel);
+    if (llmKey) await put(deps.db, "llm_key", encrypt(deps.key, llmKey));
+    return c.body(null, 204);
+  });
+
+  return app;
+}
diff --git a/server/test/settings.test.ts b/server/test/settings.test.ts
new file mode 100644
index 0000000..56832fb
--- /dev/null
+++ b/server/test/settings.test.ts
@@ -0,0 +1,58 @@
+import { describe, expect, test } from "bun:test";
+import { mkdtempSync } from "node:fs";
+import { tmpdir } from "node:os";
+import { join } from "node:path";
+import { eq } from "drizzle-orm";
+import { createApp } from "../src/app";
+import { openDb } from "../src/db";
+import { settings } from "../src/db/schema";
+import { loadOrCreateKey } from "../src/lib/crypto";
+
+async function authedApp() {
+  const dir = mkdtempSync(join(tmpdir(), "helios-"));
+  const db = openDb(dir);
+  const app = createApp({ db, key: loadOrCreateKey(dir) });
+  const j = (b: unknown) => ({ method: "POST", headers: { "content-type": "application/json" }, body: JSON.stringify(b) });
+  await app.request("/api/setup", j({ password: "hunter2hunter2" }));
+  const cookie = (await app.request("/api/login", j({ password: "hunter2hunter2" }))).headers.get("set-cookie")!;
+  return { app, db, cookie };
+}
+
+describe("settings", () => {
+  test("defaults, update, masked key, encrypted at rest", async () => {
+    const { app, db, cookie } = await authedApp();
+    const h = { cookie };
+
+    let s = await (await app.request("/api/settings", { headers: h })).json();
+    expect(s).toEqual({
+      llmBaseUrl: "https://openrouter.ai/api/v1",
+      llmModel: "anthropic/claude-sonnet-4.5",
+      llmKeyMasked: null,
+    });
+
+    const put = await app.request("/api/settings", {
+      method: "PUT",
+      headers: { ...h, "content-type": "application/json" },
+      body: JSON.stringify({ llmKey: "sk-or-v1-supersecret-abcd", llmModel: "meta-llama/llama-4" }),
+    });
+    expect(put.status).toBe(204);
+
+    s = await (await app.request("/api/settings", { headers: h })).json();
+    expect(s.llmKeyMasked).toBe("…abcd");
+    expect(s.llmModel).toBe("meta-llama/llama-4");
+
+    const raw = (await db.select().from(settings).where(eq(settings.key, "llm_key"))).at(0)!.value;
+    expect(raw.startsWith("enc:")).toBe(true);
+    expect(raw).not.toContain("supersecret");
+  });
+
+  test("rejects bad body", async () => {
+    const { app, cookie } = await authedApp();
+    const r = await app.request("/api/settings", {
+      method: "PUT",
+      headers: { cookie, "content-type": "application/json" },
+      body: JSON.stringify({ llmBaseUrl: "not a url" }),
+    });
+    expect(r.status).toBe(400);
+  });
+});
diff --git a/shared/src/types.ts b/shared/src/types.ts
index c0b2eae..1557441 100644
--- a/shared/src/types.ts
+++ b/shared/src/types.ts
@@ -6,3 +6,17 @@ export type HealthResponse = z.infer<typeof HealthResponse>;
 export const PasswordBody = z.object({ password: z.string().min(8).max(200) });
 export const MeResponse = z.object({ needsSetup: z.boolean(), authenticated: z.boolean() });
 export type MeResponse = z.infer<typeof MeResponse>;
+
+export const SettingsResponse = z.object({
+  llmBaseUrl: z.string(),
+  llmModel: z.string(),
+  llmKeyMasked: z.string().nullable(),
+});
+export type SettingsResponse = z.infer<typeof SettingsResponse>;
+
+export const SettingsUpdate = z.object({
+  llmBaseUrl: z.string().url().optional(),
+  llmModel: z.string().min(1).optional(),
+  llmKey: z.string().min(1).optional(),
+});
+export type SettingsUpdate = z.infer<typeof SettingsUpdate>;