diff --git a/src/i18n/serializer.test.ts b/src/i18n/serializer.test.ts
new file mode 100644
index 00000000..1016a935
--- /dev/null
+++ b/src/i18n/serializer.test.ts
@@ -0,0 +1,41 @@
+import { afterEach, describe, expect, it } from "vitest";
+import { createI18nInstance } from "./config.js";
+import { serializeI18nForHydration, hydrateI18nFromWindow } from "./serializer.js";
+
+describe("serializeI18nForHydration", () => {
+  it("returns a JSON string containing locale and resources", async () => {
+    const i18n = await createI18nInstance({ locale: "ru" });
+    const json = serializeI18nForHydration(i18n);
+    const parsed = JSON.parse(json);
+    expect(parsed.locale).toBe("ru");
+    expect(parsed.resources).toBeDefined();
+    expect(parsed.resources.common).toBeDefined();
+    expect(parsed.resources.common["BOARD"]).toBeDefined();
+  });
+});
+
+describe("hydrateI18nFromWindow", () => {
+  afterEach(() => {
+    // Clean up the global
+    if (typeof globalThis !== "undefined") {
+      delete (globalThis as Record<string, unknown>).__I18N__;
+    }
+  });
+
+  it("roundtrips: serialize → inject into globalThis.__I18N__ → hydrate", async () => {
+    const original = await createI18nInstance({ locale: "ru" });
+    const json = serializeI18nForHydration(original);
+    const payload = JSON.parse(json);
+
+    // Simulate the SSR injection into the global scope
+    (globalThis as Record<string, unknown>).__I18N__ = payload;
+
+    const hydrated = await hydrateI18nFromWindow();
+    expect(hydrated.language).toBe("ru");
+    expect(hydrated.t("BOARD.DEPARTURE")).toBe(original.t("BOARD.DEPARTURE"));
+  });
+
+  it("throws if __I18N__ is not set", async () => {
+    await expect(hydrateI18nFromWindow()).rejects.toThrow(/__I18N__/);
+  });
+});
diff --git a/src/i18n/serializer.ts b/src/i18n/serializer.ts
new file mode 100644
index 00000000..41371dc8
--- /dev/null
+++ b/src/i18n/serializer.ts
@@ -0,0 +1,61 @@
+import type i18next from "i18next";
+import { createI18nInstance } from "./config.js";
+import { isLanguage } from "./resolver.js";
+
+interface I18nPayload {
+  locale: string;
+  resources: Record<string, Record<string, unknown>>;
+}
+
+/**
+ * Serializes the loaded locale bundle from an i18next instance into a JSON
+ * string suitable for injection into the SSR HTML under `window.__I18N__`.
+ */
+export function serializeI18nForHydration(i18n: typeof i18next): string {
+  const locale = i18n.language;
+  const resources: Record<string, Record<string, unknown>> = {};
+
+  const namespaces = Array.isArray(i18n.options.ns)
+    ? i18n.options.ns
+    : ([i18n.options.ns].filter(Boolean) as string[]);
+
+  for (const ns of namespaces) {
+    const bundle = i18n.getResourceBundle(locale, ns) as
+      | Record<string, unknown>
+      | undefined;
+    if (bundle) {
+      resources[ns] = bundle;
+    }
+  }
+
+  const payload: I18nPayload = { locale, resources };
+  return JSON.stringify(payload);
+}
+
+/**
+ * Rehydrates an i18next instance from the SSR-injected `window.__I18N__` payload.
+ * Called once on the client before React hydration begins.
+ */
+export async function hydrateI18nFromWindow(): Promise<typeof i18next> {
+  const raw = (globalThis as Record<string, unknown>).__I18N__ as
+    | I18nPayload
+    | undefined;
+  if (!raw) {
+    throw new Error(
+      "Cannot hydrate i18n: globalThis.__I18N__ is not set. " +
+        "Ensure the SSR payload includes the serialized i18n data.",
+    );
+  }
+
+  const locale = raw.locale;
+  if (!isLanguage(locale)) {
+    throw new Error(
+      `Cannot hydrate i18n: invalid locale "${locale}" in __I18N__ payload.`,
+    );
+  }
+
+  return createI18nInstance({
+    locale,
+    initialResources: raw.resources,
+  });
+}