diff --git a/frontend/src/tests/Sidebar.test.tsx b/frontend/src/tests/Sidebar.test.tsx
new file mode 100644
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--- /dev/null
+++ b/frontend/src/tests/Sidebar.test.tsx
@@ -0,0 +1,145 @@
+import { render, screen } from "@testing-library/react";
+import userEvent from "@testing-library/user-event";
+import { MemoryRouter } from "react-router-dom";
+import { describe, it, expect, vi } from "vitest";
+import Sidebar from "../components/sidebar/Sidebar";
+import { ROUTES } from "../AppRoutes";
+
+// mock i18next to return the key as the translation
+vi.mock("react-i18next", () => ({
+  useTranslation: () => ({
+    t: (key: string) => key,
+  }),
+}));
+
+// replace the child components with simple stand-ins. They have their own
+// tests (or are not part of what we test here), and this keeps the Sidebar
+// test focused on the Sidebar itself
+vi.mock("../components/languageSwitcher/LanguageSwitcher", () => ({
+  default: () => <div data-testid="language-switcher" />,
+}));
+
+vi.mock("../components/ThemeToggle", () => ({
+  default: () => <div data-testid="theme-toggle" />,
+}));
+
+// render the Sidebar inside a router, as NavLink needs one.
+// "route" is the page the router starts on, which decides the active link
+function renderSidebar({
+  isOpen = false,
+  route = ROUTES.plantMatch,
+}: { isOpen?: boolean; route?: string } = {}) {
+  const user = userEvent.setup();
+  const onCloseSidebar = vi.fn(); // a spy that records how many times it is called
+
+  render(
+    <MemoryRouter initialEntries={[route]}>
+      <Sidebar isOpen={isOpen} onCloseSidebar={onCloseSidebar} />
+    </MemoryRouter>,
+  );
+
+  return { user, onCloseSidebar };
+}
+
+describe("Sidebar", () => {
+  // sidebar displays the title, links to all pages, language switcher, and theme toggle
+  it("displays the title", () => {
+    renderSidebar();
+
+    expect(screen.getByText("sidebar.title")).toBeInTheDocument();
+  });
+
+  it("displays navigation links to all pages", () => {
+    renderSidebar();
+
+    expect(
+      screen.getByRole("link", { name: "sidebar.plantMatch" }),
+    ).toHaveAttribute("href", ROUTES.plantMatch);
+    expect(
+      screen.getByRole("link", { name: "sidebar.explorePlants" }),
+    ).toHaveAttribute("href", ROUTES.explore);
+    expect(
+      screen.getByRole("link", { name: "sidebar.myMatches" }),
+    ).toHaveAttribute("href", ROUTES.myMatches);
+    expect(
+      screen.getByRole("link", { name: "sidebar.profile" }),
+    ).toHaveAttribute("href", ROUTES.profile);
+  });
+
+  it("displays the language switcher and theme toggle", () => {
+    renderSidebar();
+
+    expect(screen.getByTestId("language-switcher")).toBeInTheDocument();
+    expect(screen.getByTestId("theme-toggle")).toBeInTheDocument();
+  });
+
+  // on mobile the sidebar is closed after the user picks a page or clicks the close button. The onCloseSidebar prop is called in both cases.
+  it("closes the sidebar when a link is clicked", async () => {
+    const { user, onCloseSidebar } = renderSidebar();
+
+    await user.click(screen.getByRole("link", { name: "sidebar.profile" }));
+
+    expect(onCloseSidebar).toHaveBeenCalledTimes(1);
+  });
+
+  it("closes the sidebar when the close button is clicked", async () => {
+    const { user, onCloseSidebar } = renderSidebar();
+
+    await user.click(screen.getByRole("button", { name: "Close menu" }));
+
+    expect(onCloseSidebar).toHaveBeenCalledTimes(1);
+  });
+
+  // data-open controls whether the sidebar is shown or hidden on mobile,
+  // so it should be "true" when isOpen is true and "false" when it is false.
+  // The <aside> element has the role "complementary"
+  it("reflects the open state in the data-open attribute", () => {
+    const { unmount } = render(
+      <MemoryRouter>
+        <Sidebar isOpen={true} onCloseSidebar={vi.fn()} />
+      </MemoryRouter>,
+    );
+    expect(screen.getByRole("complementary")).toHaveAttribute(
+      "data-open",
+      "true",
+    );
+    unmount();
+
+    render(
+      <MemoryRouter>
+        <Sidebar isOpen={false} onCloseSidebar={vi.fn()} />
+      </MemoryRouter>,
+    );
+    expect(screen.getByRole("complementary")).toHaveAttribute(
+      "data-open",
+      "false",
+    );
+  });
+
+  // the <nav> has an aria-label, so screen readers can announce it and let users jump to it
+  it("has an accessible navigation landmark", () => {
+    renderSidebar();
+
+    expect(
+      screen.getByRole("navigation", { name: "Sidebar navigation" }),
+    ).toBeInTheDocument();
+  });
+
+  // the router is set to the Explore page, so only that link should get aria-current="page"
+  it("marks only the link for the current page as active", () => {
+    renderSidebar({ route: ROUTES.explore });
+
+    expect(
+      screen.getByRole("link", { name: "sidebar.explorePlants" }),
+    ).toHaveAttribute("aria-current", "page");
+    expect(
+      screen.getByRole("link", { name: "sidebar.plantMatch" }),
+    ).not.toHaveAttribute("aria-current");
+    expect(
+      screen.getByRole("link", { name: "sidebar.myMatches" }),
+    ).not.toHaveAttribute("aria-current");
+    expect(
+      screen.getByRole("link", { name: "sidebar.profile" }),
+    ).not.toHaveAttribute("aria-current");
+  });
+});