diff --git a/.changeset/mermaid-fullscreen-download.md b/.changeset/mermaid-fullscreen-download.md
new file mode 100644
index 00000000..98183958
--- /dev/null
+++ b/.changeset/mermaid-fullscreen-download.md
@@ -0,0 +1,5 @@
+---
+"streamdown": patch
+---
+
+fix(mermaid): render the download control inside the Mermaid fullscreen portal so it's reachable when the diagram is expanded
diff --git a/packages/streamdown/__tests__/mermaid-fullscreen.test.tsx b/packages/streamdown/__tests__/mermaid-fullscreen.test.tsx
index aa72ca8e..9942fa58 100644
--- a/packages/streamdown/__tests__/mermaid-fullscreen.test.tsx
+++ b/packages/streamdown/__tests__/mermaid-fullscreen.test.tsx
@@ -2,6 +2,7 @@ import { fireEvent, render, waitFor } from "@testing-library/react";
 import { afterEach, describe, expect, it, vi } from "vitest";
 import { StreamdownContext } from "../index";
 import { PluginContext } from "../lib/plugin-context";
+import type { DiagramPlugin, MermaidInstance } from "../lib/plugin-types";
 
 // Must mock the Mermaid component that fullscreen-button imports
 vi.mock("../lib/mermaid", () => ({
@@ -12,8 +13,23 @@ vi.mock("../lib/mermaid", () => ({
   ),
 }));
 
-// Import after mock
+vi.mock("../lib/utils", async () => {
+  const actual = await vi.importActual("../lib/utils");
+  return {
+    ...actual,
+    save: vi.fn(),
+  };
+});
+
+vi.mock("../lib/mermaid/utils", () => ({
+  svgToPngBlob: vi
+    .fn()
+    .mockResolvedValue(new Blob(["png"], { type: "image/png" })),
+}));
+
 import { MermaidFullscreenButton } from "../lib/mermaid/fullscreen-button";
+// Import after mocks
+import { save } from "../lib/utils";
 
 describe("MermaidFullscreenButton", () => {
   const defaultContext = {
@@ -23,9 +39,28 @@ describe("MermaidFullscreenButton", () => {
     mode: "streaming" as const,
   };
 
-  const renderWithContext = (props: any, contextOverrides = {}) => {
+  const createMockPlugin = (
+    renderResult = { svg: "<svg><text>Chart</text></svg>" }
+  ): DiagramPlugin => {
+    const mockInstance: MermaidInstance = {
+      initialize: vi.fn(),
+      render: vi.fn().mockResolvedValue(renderResult),
+    };
+    return {
+      name: "mermaid",
+      type: "diagram",
+      language: "mermaid",
+      getMermaid: vi.fn().mockReturnValue(mockInstance),
+    };
+  };
+
+  const renderWithContext = (
+    props: any,
+    contextOverrides = {},
+    plugin: DiagramPlugin | undefined = undefined
+  ) => {
     return render(
-      <PluginContext.Provider value={{}}>
+      <PluginContext.Provider value={plugin ? { mermaid: plugin } : {}}>
         <StreamdownContext.Provider
           value={{ ...defaultContext, ...contextOverrides }}
         >
@@ -246,4 +281,67 @@ describe("MermaidFullscreenButton", () => {
       expect(mermaid?.textContent).toContain("graph TD; A-->B");
     });
   });
+
+  it("should render a working download button inside the fullscreen portal", async () => {
+    const plugin = createMockPlugin();
+    const { container } = renderWithContext(
+      { chart: "graph TD; A-->B" },
+      {},
+      plugin
+    );
+
+    const openBtn = container.querySelector('button[title="View fullscreen"]');
+    // biome-ignore lint/style/noNonNullAssertion: test assertion
+    fireEvent.click(openBtn!);
+
+    await waitFor(() => {
+      expect(document.querySelector(".fixed.inset-0")).toBeTruthy();
+    });
+
+    const downloadBtn = document.querySelector(
+      'button[title="Download diagram"]'
+    );
+    expect(downloadBtn).toBeTruthy();
+
+    // biome-ignore lint/style/noNonNullAssertion: test assertion
+    fireEvent.click(downloadBtn!);
+
+    const pngOption = await waitFor(() =>
+      document.querySelector('button[title="Download diagram as PNG"]')
+    );
+    // biome-ignore lint/style/noNonNullAssertion: test assertion
+    fireEvent.click(pngOption!);
+
+    await waitFor(() => {
+      expect(save).toHaveBeenCalledWith(
+        "diagram.png",
+        expect.any(Blob),
+        "image/png"
+      );
+    });
+
+    // Fullscreen should still be open — downloading must not close it.
+    expect(document.querySelector(".fixed.inset-0")).toBeTruthy();
+  });
+
+  it("should hide the fullscreen download button when the download control is disabled", async () => {
+    const plugin = createMockPlugin();
+    const { container } = renderWithContext(
+      { chart: "graph TD; A-->B" },
+      { controls: { mermaid: { download: false } } },
+      plugin
+    );
+
+    const openBtn = container.querySelector('button[title="View fullscreen"]');
+    // biome-ignore lint/style/noNonNullAssertion: test assertion
+    fireEvent.click(openBtn!);
+
+    await waitFor(() => {
+      expect(document.querySelector(".fixed.inset-0")).toBeTruthy();
+    });
+
+    expect(
+      document.querySelector('button[title="Download diagram"]')
+    ).toBeFalsy();
+  });
 });
diff --git a/packages/streamdown/lib/mermaid/fullscreen-button.tsx b/packages/streamdown/lib/mermaid/fullscreen-button.tsx
index 316095da..53a906b3 100644
--- a/packages/streamdown/lib/mermaid/fullscreen-button.tsx
+++ b/packages/streamdown/lib/mermaid/fullscreen-button.tsx
@@ -7,6 +7,7 @@ import { useCn } from "../prefix-context";
 import { lockBodyScroll, unlockBodyScroll } from "../scroll-lock";
 import { useTranslations } from "../translations-context";
 import { Mermaid } from ".";
+import { MermaidDownloadDropdown } from "./download-button";
 
 type MermaidFullscreenButtonProps = ComponentProps<"button"> & {
   chart: string;
@@ -42,6 +43,19 @@ export const MermaidFullscreenButton = ({
     }
     return mermaidCtl.panZoom !== false;
   })();
+  const showDownload = (() => {
+    if (typeof controlsConfig === "boolean") {
+      return controlsConfig;
+    }
+    const mermaidCtl = controlsConfig.mermaid;
+    if (mermaidCtl === false) {
+      return false;
+    }
+    if (mermaidCtl === true || mermaidCtl === undefined) {
+      return true;
+    }
+    return mermaidCtl.download !== false;
+  })();
 
   const handleToggle = () => {
     setIsFullscreen(!isFullscreen);
@@ -107,16 +121,29 @@ export const MermaidFullscreenButton = ({
               role="button"
               tabIndex={0}
             >
-              <button
+              {/* biome-ignore lint/a11y/noStaticElementInteractions: "div with role=presentation is used for event propagation control" */}
+              <div
                 className={cn(
-                  "absolute top-4 right-4 z-10 rounded-md p-2 text-muted-foreground transition-all hover:bg-muted hover:text-foreground"
+                  "absolute top-4 right-4 z-10 flex items-center gap-1"
                 )}
-                onClick={handleToggle}
-                title={t.exitFullscreen}
-                type="button"
+                onClick={(e) => e.stopPropagation()}
+                onKeyDown={(e) => e.stopPropagation()}
+                role="presentation"
               >
-                <XIcon size={20} />
-              </button>
+                {showDownload ? (
+                  <MermaidDownloadDropdown chart={chart} config={config} />
+                ) : null}
+                <button
+                  className={cn(
+                    "rounded-md p-2 text-muted-foreground transition-all hover:bg-muted hover:text-foreground"
+                  )}
+                  onClick={handleToggle}
+                  title={t.exitFullscreen}
+                  type="button"
+                >
+                  <XIcon size={20} />
+                </button>
+              </div>
               {/* biome-ignore lint/a11y/noStaticElementInteractions: "div with role=presentation is used for event propagation control" */}
               <div
                 className={cn("flex size-full items-center justify-center p-4")}