diff --git a/.changeset/fix-mermaid-hard-dep.md b/.changeset/fix-mermaid-hard-dep.md
new file mode 100644
index 00000000..fe47f067
--- /dev/null
+++ b/.changeset/fix-mermaid-hard-dep.md
@@ -0,0 +1,9 @@
+---
+"streamdown": patch
+---
+
+fix(deps): remove `mermaid` as a hard runtime dependency
+
+This patch replaces the type import with a local structural type for `MermaidConfig` so no type-level coupling to the `mermaid` package remains in the distributed typings. Users who want fully-typed mermaid config can still `import type { MermaidConfig } from 'mermaid'` themselves; the structural type is compatible.
+
+Fixes #501
diff --git a/packages/streamdown/__tests__/mermaid.test.tsx b/packages/streamdown/__tests__/mermaid.test.tsx
index 7af696fd..0cfaff03 100644
--- a/packages/streamdown/__tests__/mermaid.test.tsx
+++ b/packages/streamdown/__tests__/mermaid.test.tsx
@@ -1,12 +1,11 @@
 import { render, waitFor } from "@testing-library/react";
-import type { MermaidConfig } from "mermaid";
 import { act } from "react";
 import { beforeEach, describe, expect, it, vi } from "vitest";
 import { Mermaid } from "../lib/mermaid";
 import { MermaidDownloadDropdown } from "../lib/mermaid/download-button";
 import { MermaidFullscreenButton } from "../lib/mermaid/fullscreen-button";
 import { PluginContext } from "../lib/plugin-context";
-import type { DiagramPlugin } from "../lib/plugin-types";
+import type { DiagramPlugin, MermaidConfig } from "../lib/plugin-types";
 
 const { saveMock, mockInitialize, mockRender, mockMermaid } = vi.hoisted(() => {
   const mockInitialize = vi.fn();
diff --git a/packages/streamdown/__tests__/streamdown.test.tsx b/packages/streamdown/__tests__/streamdown.test.tsx
index 06c3c8b4..491c29dd 100644
--- a/packages/streamdown/__tests__/streamdown.test.tsx
+++ b/packages/streamdown/__tests__/streamdown.test.tsx
@@ -1,7 +1,7 @@
 import { render } from "@testing-library/react";
-import type { MermaidConfig } from "mermaid";
 import { describe, expect, it, vi } from "vitest";
 import { Streamdown } from "../index";
+import type { MermaidConfig } from "../lib/plugin-types";
 
 // Mock the dependencies
 vi.mock("../lib/markdown", () => ({
diff --git a/packages/streamdown/index.tsx b/packages/streamdown/index.tsx
index 5b8cc920..e36692da 100644
--- a/packages/streamdown/index.tsx
+++ b/packages/streamdown/index.tsx
@@ -1,6 +1,5 @@
 "use client";
 
-import type { MermaidConfig } from "mermaid";
 import {
   type ComponentProps,
   type CSSProperties,
@@ -33,7 +32,11 @@ import { hasIncompleteCodeFence, hasTable } from "./lib/incomplete-code-utils";
 import { type ExtraProps, Markdown, type Options } from "./lib/markdown";
 import { parseMarkdownIntoBlocks } from "./lib/parse-blocks";
 import { PluginContext } from "./lib/plugin-context";
-import type { PluginConfig, ThemeInput } from "./lib/plugin-types";
+import type {
+  MermaidConfig,
+  PluginConfig,
+  ThemeInput,
+} from "./lib/plugin-types";
 import { PrefixContext } from "./lib/prefix-context";
 import { preprocessCustomTags } from "./lib/preprocess-custom-tags";
 import { preprocessLiteralTagContent } from "./lib/preprocess-literal-tag-content";
diff --git a/packages/streamdown/lib/mermaid/download-button.tsx b/packages/streamdown/lib/mermaid/download-button.tsx
index 7f287f2d..c46c94e5 100644
--- a/packages/streamdown/lib/mermaid/download-button.tsx
+++ b/packages/streamdown/lib/mermaid/download-button.tsx
@@ -1,8 +1,8 @@
-import type { MermaidConfig } from "mermaid";
 import { useContext, useEffect, useRef, useState } from "react";
 import { StreamdownContext } from "../../index";
 import { useIcons } from "../icon-context";
 import { useMermaidPlugin } from "../plugin-context";
+import type { MermaidConfig } from "../plugin-types";
 import { useCn } from "../prefix-context";
 import { useTranslations } from "../translations-context";
 import { save } from "../utils";
diff --git a/packages/streamdown/lib/mermaid/fullscreen-button.tsx b/packages/streamdown/lib/mermaid/fullscreen-button.tsx
index 316095da..8491fd67 100644
--- a/packages/streamdown/lib/mermaid/fullscreen-button.tsx
+++ b/packages/streamdown/lib/mermaid/fullscreen-button.tsx
@@ -1,8 +1,8 @@
-import type { MermaidConfig } from "mermaid";
 import { type ComponentProps, useContext, useEffect, useState } from "react";
 import { createPortal } from "react-dom";
 import { StreamdownContext } from "../../index";
 import { useIcons } from "../icon-context";
+import type { MermaidConfig } from "../plugin-types";
 import { useCn } from "../prefix-context";
 import { lockBodyScroll, unlockBodyScroll } from "../scroll-lock";
 import { useTranslations } from "../translations-context";
diff --git a/packages/streamdown/lib/mermaid/index.tsx b/packages/streamdown/lib/mermaid/index.tsx
index 3f483863..4f4408cf 100644
--- a/packages/streamdown/lib/mermaid/index.tsx
+++ b/packages/streamdown/lib/mermaid/index.tsx
@@ -1,8 +1,8 @@
-import type { MermaidConfig } from "mermaid";
 import { useContext, useEffect, useState } from "react";
 import { useDeferredRender } from "../../hooks/use-deferred-render";
 import { StreamdownContext } from "../../index";
 import { useMermaidPlugin } from "../plugin-context";
+import type { MermaidConfig } from "../plugin-types";
 import { useCn } from "../prefix-context";
 import { PanZoom } from "./pan-zoom";
 
diff --git a/packages/streamdown/lib/plugin-types.ts b/packages/streamdown/lib/plugin-types.ts
index 01c45b64..807341f7 100644
--- a/packages/streamdown/lib/plugin-types.ts
+++ b/packages/streamdown/lib/plugin-types.ts
@@ -1,4 +1,3 @@
-import type { MermaidConfig } from "mermaid";
 import type React from "react";
 import type { Pluggable } from "unified";
 import type {
@@ -80,21 +79,44 @@ export interface CodeHighlighterPlugin {
 }
 
 /**
- * Mermaid instance interface
+ * Structural type for Mermaid configuration pass-through.
+ * Avoids a hard dependency on the "mermaid" package in the core bundle.
+ */
+export type MermaidConfig = {
+  fontFamily?: string;
+  securityLevel?: string;
+  startOnLoad?: boolean;
+  suppressErrorRendering?: boolean;
+  theme?: string;
+  themeCSS?: string;
+  themeVariables?: Record<string, unknown>;
+  // biome-ignore lint/suspicious/noExplicitAny: open pass-through for mermaid options
+} & Record<string, any>;
+
+/**
+ * Mermaid instance interface.
+ *
+ * Method syntax is intentional: parameter types stay bivariant so
+ * `@streamdown/mermaid` (which uses mermaid's narrower `MermaidConfig`) remains
+ * assignable without a core type dependency on the mermaid package.
  */
 export interface MermaidInstance {
-  initialize: (config: MermaidConfig) => void;
-  render: (id: string, source: string) => Promise<{ svg: string }>;
+  initialize(config: MermaidConfig): void;
+  render(id: string, source: string): Promise<{ svg: string }>;
 }
 
 /**
- * Plugin for diagram rendering (Mermaid)
+ * Plugin for diagram rendering (Mermaid).
+ *
+ * Method syntax on `getMermaid` matches `CodeHighlighterPlugin` — keeps plugin
+ * implementations with narrower config types assignable under
+ * `strictFunctionTypes`.
  */
 export interface DiagramPlugin {
   /**
    * Get the mermaid instance (initialized with optional config)
    */
-  getMermaid: (config?: MermaidConfig) => MermaidInstance;
+  getMermaid(config?: MermaidConfig): MermaidInstance;
   /**
    * Language identifier for code blocks
    */
diff --git a/packages/streamdown/package.json b/packages/streamdown/package.json
index cecaba75..f54d7152 100644
--- a/packages/streamdown/package.json
+++ b/packages/streamdown/package.json
@@ -73,7 +73,6 @@
     "remend": "workspace:*",
     "tailwind-merge": "^3.4.0",
     "unified": "^11.0.5",
-    "mermaid": "^11.12.2",
     "unist-util-visit": "^5.0.0",
     "unist-util-visit-parents": "^6.0.0"
   }
diff --git a/pnpm-lock.yaml b/pnpm-lock.yaml
index 2e7ac6d0..c10a209e 100644
--- a/pnpm-lock.yaml
+++ b/pnpm-lock.yaml
@@ -292,9 +292,6 @@ importers:
       marked:
         specifier: ^17.0.1
         version: 17.0.1
-      mermaid:
-        specifier: ^11.12.2
-        version: 11.12.2
       react:
         specifier: ^18.0.0 || ^19.0.0
         version: 19.2.3