diff --git a/src/api/auth.ts b/src/api/auth.ts
index 915fe27..d7cfbce 100644
--- a/src/api/auth.ts
+++ b/src/api/auth.ts
@@ -1,28 +1,48 @@
+import { LoginRequest, LoginResponse, SignUpRequest } from "./auth.types";
 import { apiFetch } from "./client";
 
-export type LoginRequest = {
-  loginId: string;
-  password: string;
-};
-
-export async function loginApi(body: LoginRequest): Promise<string> {
+export async function loginApi(body: LoginRequest): Promise<LoginResponse> {
   try {
     const res = await apiFetch("/api/v1/auth/login", {
       method: "POST",
       body: JSON.stringify(body),
     });
 
-    console.log("res status:", res.status);
-    const json = (await res.json()) as any;
+    const json = (await res.json()) as LoginResponse;
 
-    const token: string | undefined =
-      json?.data?.accessToken ?? json?.accessToken ?? json?.data?.token;
+    const accessToken = json?.data?.accessToken;
+    const refreshToken = json?.data?.refreshToken;
 
-    if (!token) throw new Error("No accessToken in response");
+    if (!accessToken || !refreshToken) {
+      throw new Error("Invalid login response: missing token");
+    }
 
-    return token;
+    return json;
   } catch (e) {
     console.error("로그인 api 실패:", e);
     throw e;
   }
 }
+
+export async function signUpApi(body: SignUpRequest): Promise<LoginResponse> {
+  try {
+    const res = await apiFetch("/api/v1/auth/register", {
+      method: "POST",
+      body: JSON.stringify(body),
+    });
+
+    const json = (await res.json()) as LoginResponse;
+
+    const accessToken = json?.data?.accessToken;
+    const refreshToken = json?.data?.refreshToken;
+
+    if (!accessToken || !refreshToken) {
+      throw new Error("Invalid login response: missing token");
+    }
+
+    return json;
+  } catch (e) {
+    console.error("회원가입 api 실패:", e);
+    throw e;
+  }
+}
diff --git a/src/api/auth.types.ts b/src/api/auth.types.ts
new file mode 100644
index 0000000..ac3fe68
--- /dev/null
+++ b/src/api/auth.types.ts
@@ -0,0 +1,18 @@
+import type { ApiResponse } from "./response.types";
+
+// 로그인 api
+export type LoginRequest = {
+  loginId: string;
+  password: string;
+};
+
+export type LoginData = {
+  accessToken: string;
+  refreshToken: string;
+};
+
+export type LoginResponse = ApiResponse<LoginData>;
+
+// 회원가입 api
+export type SignUpRequest = { loginId: string; password: string };
+export type SignUpResponse = { userId: number };
diff --git a/src/api/client.ts b/src/api/client.ts
index 2ac53e3..3392e7a 100644
--- a/src/api/client.ts
+++ b/src/api/client.ts
@@ -1,24 +1,43 @@
-function getApiBaseUrl() {
+import { AppError, type ApiErrorPayload } from "./response.types";
+
+function getApiBaseUrl(): string {
   const url = process.env.EXPO_PUBLIC_API_BASE_URL?.trim();
-  if (!url) throw new Error("Missing EXPO_PUBLIC_API_BASE_URL");
+  if (!url) throw new AppError("Missing EXPO_PUBLIC_API_BASE_URL");
   return url.replace(/\/$/, "");
 }
 
 export async function apiFetch(path: string, init?: RequestInit) {
-  const API_BASE_URL = getApiBaseUrl();
+  const baseUrl = getApiBaseUrl();
   const safePath = path.startsWith("/") ? path : `/${path}`;
 
-  const res = await fetch(`${API_BASE_URL}${safePath}`, {
-    ...init,
-    headers: {
-      "Content-Type": "application/json",
-      ...(init?.headers ?? {}),
-    },
-  });
+  let res: Response;
+  try {
+    res = await fetch(`${baseUrl}${safePath}`, {
+      ...init,
+      headers: {
+        "Content-Type": "application/json",
+        ...(init?.headers ?? {}),
+      },
+    });
+  } catch (e) {
+    throw new AppError("Network error", { httpStatus: 0 });
+  }
 
   if (!res.ok) {
     const text = await res.text().catch(() => "");
-    throw new Error(`HTTP ${res.status} ${res.statusText} ${text}`);
+    let apiPayload: ApiErrorPayload | undefined;
+
+    try {
+      const parsed = text ? JSON.parse(text) : null;
+      apiPayload = parsed?.data as ApiErrorPayload | undefined;
+    } catch {
+      // ignore
+    }
+
+    throw new AppError(apiPayload?.message ?? `HTTP ${res.status}`, {
+      httpStatus: res.status,
+      api: apiPayload,
+    });
   }
 
   return res;
diff --git a/src/api/response.types.ts b/src/api/response.types.ts
new file mode 100644
index 0000000..7274721
--- /dev/null
+++ b/src/api/response.types.ts
@@ -0,0 +1,31 @@
+export type ApiResponse<T> = {
+  data: T;
+};
+
+export type ApiErrorPayload = {
+  title: string;
+  errorCode: string;
+  message: string;
+  code: number;
+  status: string;
+  instance?: string;
+};
+
+export class AppError extends Error {
+  readonly kind = "AppError";
+  readonly httpStatus?: number;
+  readonly api?: ApiErrorPayload;
+
+  constructor(
+    message: string,
+    opts?: { httpStatus?: number; api?: ApiErrorPayload }
+  ) {
+    super(message);
+    this.httpStatus = opts?.httpStatus;
+    this.api = opts?.api;
+  }
+}
+
+export function isAppError(e: unknown): e is AppError {
+  return e instanceof AppError;
+}
diff --git a/src/components/ui/AppModal.tsx b/src/components/ui/AppModal.tsx
new file mode 100644
index 0000000..26ae12b
--- /dev/null
+++ b/src/components/ui/AppModal.tsx
@@ -0,0 +1,84 @@
+import React from "react";
+import { Modal, Pressable, StyleSheet, View } from "react-native";
+import { Theme } from "../../theme/theme";
+import AppText from "./AppText";
+import Button from "./Button";
+
+type Props = {
+  visible: boolean;
+  title: string;
+  message?: string;
+  primaryText?: string;
+  onPrimaryPress?: () => void;
+  onClose: () => void;
+};
+
+export default function AppModal({
+  visible,
+  title,
+  message,
+  primaryText = "OK",
+  onPrimaryPress,
+  onClose,
+}: Props) {
+  return (
+    <Modal
+      visible={visible}
+      transparent
+      animationType="fade"
+      onRequestClose={onClose}
+    >
+      <Pressable style={styles.backdrop} onPress={onClose}>
+        <Pressable style={styles.card} onPress={() => {}}>
+          <AppText variant="title" style={styles.title}>
+            {title}
+          </AppText>
+
+          {message ? (
+            <AppText variant="muted" style={styles.message}>
+              {message}
+            </AppText>
+          ) : null}
+
+          <View style={{ height: Theme.Spacing.lg }} />
+
+          <Button
+            title={primaryText}
+            onPress={onPrimaryPress ?? onClose}
+            style={styles.button}
+          />
+        </Pressable>
+      </Pressable>
+    </Modal>
+  );
+}
+
+const styles = StyleSheet.create({
+  backdrop: {
+    flex: 1,
+    justifyContent: "center",
+    padding: Theme.Spacing.xl,
+    backgroundColor: "rgba(0,0,0,0.55)",
+  },
+  card: {
+    width: "100%",
+    maxWidth: 420, // 큰 화면에서 너무 넓어지지 않게
+    alignSelf: "center",
+    borderRadius: Theme.Radius.lg,
+    backgroundColor: Theme.Colors.surface,
+    padding: Theme.Spacing.xl,
+    borderWidth: 1,
+    borderColor: Theme.Colors.border,
+  },
+  title: {
+    // textAlign: "center",
+  },
+  message: {
+    // textAlign: "center",
+    marginTop: Theme.Spacing.md,
+    lineHeight: 20,
+  },
+  button: {
+    minHeight: 48,
+  },
+});
diff --git a/src/screens/SignUpScreen.tsx b/src/screens/SignUpScreen.tsx
index fe705c9..523d3da 100644
--- a/src/screens/SignUpScreen.tsx
+++ b/src/screens/SignUpScreen.tsx
@@ -2,18 +2,25 @@ import { useNavigation } from "@react-navigation/native";
 import React, { useMemo, useState } from "react";
 import { StyleSheet, View } from "react-native";
 import { SafeAreaView } from "react-native-safe-area-context";
+import { isAppError } from "../api/response.types";
+import AppModal from "../components/ui/AppModal";
 import AppText from "../components/ui/AppText";
 import Button from "../components/ui/Button";
 import Input from "../components/ui/Input";
+import { useAuth } from "../store/auth";
 import { Theme } from "../theme/theme";
 
 export default function SignUpScreen() {
   const navigation = useNavigation();
+  const auth = useAuth();
 
   const [loginId, setLoginId] = useState("");
   const [password, setPassword] = useState("");
   const [passwordConfirm, setPasswordConfirm] = useState("");
 
+  const [dupModalOpen, setDupModalOpen] = useState(false);
+  const [dupMessage, setDupMessage] = useState("");
+
   const { canSubmit, errorText } = useMemo(() => {
     const idOk = loginId.trim().length >= 4;
     const pwOk = password.length >= 8;
@@ -33,7 +40,16 @@ export default function SignUpScreen() {
   }, [loginId, password, passwordConfirm]);
 
   const onSignUp = async () => {
-    navigation.goBack();
+    try {
+      await auth.signUp(loginId, password);
+    } catch (e) {
+      if (isAppError(e) && e.api?.errorCode === "AUTH-001") {
+        setDupMessage(e.api.message); // "이미 존재하는 loginId..."
+        setDupModalOpen(true);
+        return;
+      }
+      throw e;
+    }
   };
 
   return (
@@ -98,6 +114,13 @@ export default function SignUpScreen() {
           variant="secondary"
           onPress={() => navigation.goBack()}
         />
+
+        <AppModal
+          visible={dupModalOpen}
+          title="That username is taken"
+          message="Please choose another."
+          onClose={() => setDupModalOpen(false)}
+        />
       </View>
     </SafeAreaView>
   );
diff --git a/src/store/auth.tsx b/src/store/auth.tsx
index a424a8e..d4317b6 100644
--- a/src/store/auth.tsx
+++ b/src/store/auth.tsx
@@ -7,16 +7,18 @@ import React, {
   useMemo,
   useState,
 } from "react";
-import { loginApi } from "../api/auth";
+import { loginApi, signUpApi } from "../api/auth";
 
 const ACCESS_TOKEN_KEY = "accessToken";
+const REFRESH_TOKEN_KEY = "refreshToken";
 
 export type AuthContextValue = {
   accessToken: string | null;
   isAuthed: boolean;
-  isHydrating: boolean; // 앱 시작 시 저장소에서 토큰 읽는 중인지
+  isHydrating: boolean;
   signIn: (loginId: string, password: string) => Promise<void>;
   signOut: () => Promise<void>;
+  signUp: (loginId: string, password: string) => Promise<void>;
 };
 
 const AuthContext = createContext<AuthContextValue | null>(null);
@@ -34,9 +36,10 @@ export function AuthProvider({ children }: { children: React.ReactNode }) {
   }, []);
 
   const signIn = useCallback(async (loginId: string, password: string) => {
-    const token = await loginApi({ loginId, password });
-    setAccessToken(token);
-    await AsyncStorage.setItem(ACCESS_TOKEN_KEY, token);
+    const response = await loginApi({ loginId, password });
+    setAccessToken(response.data.accessToken);
+    await AsyncStorage.setItem(ACCESS_TOKEN_KEY, response.data.accessToken);
+    await AsyncStorage.setItem(REFRESH_TOKEN_KEY, response.data.accessToken);
   }, []);
 
   const signOut = useCallback(async () => {
@@ -44,6 +47,13 @@ export function AuthProvider({ children }: { children: React.ReactNode }) {
     await AsyncStorage.removeItem(ACCESS_TOKEN_KEY);
   }, []);
 
+  const signUp = useCallback(async (loginId: string, password: string) => {
+    const response = await signUpApi({ loginId, password });
+    setAccessToken(response.data.accessToken);
+    await AsyncStorage.setItem(ACCESS_TOKEN_KEY, response.data.accessToken);
+    await AsyncStorage.setItem(REFRESH_TOKEN_KEY, response.data.accessToken);
+  }, []);
+
   const value = useMemo<AuthContextValue>(
     () => ({
       accessToken,
@@ -51,6 +61,7 @@ export function AuthProvider({ children }: { children: React.ReactNode }) {
       isHydrating,
       signIn,
       signOut,
+      signUp,
     }),
     [accessToken, isHydrating, signIn, signOut]
   );