diff --git a/lib/index.ts b/lib/index.ts
index 7821adf..4b00a64 100644
--- a/lib/index.ts
+++ b/lib/index.ts
@@ -47,6 +47,12 @@ const addon = bindings<{
   databaseInitTokenizer(db: NativeDatabase): void;
   databaseExec(db: NativeDatabase, query: string): void;
   databaseClose(db: NativeDatabase): void;
+  databaseCreateFunction(
+    db: NativeDatabase,
+    name: string,
+    fn: (...args: ReadonlyArray<unknown>) => void,
+    bigint: boolean,
+  ): void;
 
   signalTokenize(value: string): Array<string>;
 
@@ -116,6 +122,14 @@ export type RowType<Options extends StatementOptions> = Options extends {
   ? SqliteValue<Options>
   : Record<string, SqliteValue<Options>>;
 
+export type FunctionOptions = Readonly<{
+  /**
+   * If `true` - all integers passed to the fucntion will be big
+   * integers instead of regular (floating-point) numbers.
+   */
+  bigint?: boolean;
+}>;
+
 /**
  * A compiled SQL statement class.
  */
@@ -452,6 +466,35 @@ export default class Database {
     addon.databaseExec(this.#native, sql);
   }
 
+  /**
+   * Create custom SQL function with a given `name`.
+   *
+   * @param name - name of the function
+   * @param fn - function implementation
+   * @param options - function options.
+   */
+  public createFunction(
+    name: string,
+    fn: (...args: ReadonlyArray<unknown>) => void,
+    options: FunctionOptions = {},
+  ): void {
+    if (this.#native === undefined) {
+      throw new Error('Database closed');
+    }
+    if (typeof name !== 'string') {
+      throw new TypeError('Invalid name argument');
+    }
+    if (typeof fn !== 'function') {
+      throw new TypeError('Invalid fn argument');
+    }
+    addon.databaseCreateFunction(
+      this.#native,
+      name,
+      fn,
+      options.bigint === true,
+    );
+  }
+
   /**
    * Compile a single SQL statement.
    *
diff --git a/src/addon.cc b/src/addon.cc
index 818e562..1f5353e 100644
--- a/src/addon.cc
+++ b/src/addon.cc
@@ -74,6 +74,90 @@ static Napi::Value SignalTokenize(const Napi::CallbackInfo& info) {
   return result;
 }
 
+// Functions
+
+class FunctionWrap {
+ public:
+  FunctionWrap(Napi::Function fn, bool is_bigint) : is_bigint_(is_bigint) {
+    fn_.Reset(fn, 1);
+  }
+
+  static void Run(sqlite3_context* ctx, int argc, sqlite3_value** argv) {
+    auto wrap = static_cast<FunctionWrap*>(sqlite3_user_data(ctx));
+
+    wrap->Call(ctx, argc, argv);
+  }
+
+  static void Final(void* p_app) { delete static_cast<FunctionWrap*>(p_app); }
+
+ protected:
+  void Call(sqlite3_context* ctx, int argc, sqlite3_value** argv) {
+    auto env = fn_.Env();
+    Napi::HandleScope scope(env);
+
+    assert(argc >= 0);
+
+    auto args = std::vector<Napi::Value>(static_cast<size_t>(argc));
+
+    for (int i = 0; i < argc; i++) {
+      args[i] = TranslateValue(argv[i]);
+    }
+
+    auto result = fn_.Value().Call(args);
+
+    // Ignore exceptions
+    if (result.IsEmpty()) {
+      auto e = env.GetAndClearPendingException();
+      sqlite3_result_error(ctx, e.Message().c_str(), SQLITE_ERROR);
+    } else if (result.IsUndefined()) {
+      sqlite3_result_null(ctx);
+    } else {
+      sqlite3_result_error(ctx, "Function must not return a value",
+                           SQLITE_ERROR);
+    }
+  }
+
+  Napi::Value TranslateValue(sqlite3_value* value) {
+    auto env = fn_.Env();
+    int type = sqlite3_value_type(value);
+    switch (type) {
+      case SQLITE_INTEGER: {
+        auto val = sqlite3_value_int64(value);
+        if (is_bigint_) {
+          return Napi::BigInt::New(env, static_cast<int64_t>(val));
+        }
+        if (static_cast<int64_t>(INT32_MIN) <= val &&
+            val <= static_cast<int64_t>(INT32_MAX)) {
+          napi_value n_value;
+          NAPI_THROW_IF_FAILED(
+              env, napi_create_int32(env, static_cast<int32_t>(val), &n_value),
+              Napi::Value());
+          return Napi::Value(env, n_value);
+        } else {
+          return Napi::Number::New(env, val);
+        }
+      }
+      case SQLITE_TEXT:
+        return Napi::String::New(
+            env, reinterpret_cast<const char*>(sqlite3_value_text(value)),
+            sqlite3_value_bytes(value));
+      case SQLITE_FLOAT:
+        return Napi::Number::New(env, sqlite3_value_double(value));
+      case SQLITE_BLOB:
+        return Napi::Buffer<uint8_t>::Copy(
+            env, reinterpret_cast<const uint8_t*>(sqlite3_value_blob(value)),
+            sqlite3_value_bytes(value));
+      case SQLITE_NULL:
+        return env.Null();
+    }
+    return Napi::Value();
+  }
+
+ private:
+  Napi::Reference<Napi::Function> fn_;
+  bool is_bigint_;
+};
+
 // Global Settings
 
 thread_local Napi::Reference<Napi::Function> logger_fn_;
@@ -150,6 +234,8 @@ Napi::Object Database::Init(Napi::Env env, Napi::Object exports) {
       Napi::Function::New(env, &Database::InitTokenizer);
   exports["databaseClose"] = Napi::Function::New(env, &Database::Close);
   exports["databaseExec"] = Napi::Function::New(env, &Database::Exec);
+  exports["databaseCreateFunction"] =
+      Napi::Function::New(env, &Database::CreateFunction);
   return exports;
 }
 
@@ -289,6 +375,42 @@ Napi::Value Database::Exec(const Napi::CallbackInfo& info) {
   return Napi::Value();
 }
 
+Napi::Value Database::CreateFunction(const Napi::CallbackInfo& info) {
+  auto env = info.Env();
+
+  auto db = FromExternal(info[0]);
+  auto name = info[1].As<Napi::String>();
+  auto fn = info[2].As<Napi::Function>();
+  auto is_bigint = info[3].As<Napi::Boolean>();
+
+  assert(name.IsString());
+  assert(fn.IsFunction());
+  assert(is_bigint.IsBoolean());
+
+  if (db == nullptr) {
+    return Napi::Value();
+  }
+
+  if (db->handle_ == nullptr) {
+    NAPI_THROW(Napi::Error::New(env, "Database closed"), Napi::Value());
+  }
+
+  auto name_utf8 = name.Utf8Value();
+
+  auto fn_wrap = new FunctionWrap(fn, is_bigint);
+
+  int r = sqlite3_create_function_v2(db->handle_, name_utf8.c_str(), -1,
+                                     SQLITE_UTF8,  // TODO(indutny): or UTF16?
+                                     fn_wrap, FunctionWrap::Run, nullptr,
+                                     nullptr, FunctionWrap::Final);
+
+  if (r != SQLITE_OK) {
+    delete fn_wrap;
+    return db->ThrowSqliteError(env, r);
+  }
+  return Napi::Value();
+}
+
 Napi::Value Database::ThrowSqliteError(Napi::Env env, int error) {
   assert(handle_ != nullptr);
   const char* msg = sqlite3_errmsg(handle_);
diff --git a/src/addon.h b/src/addon.h
index 2d76f4b..800aff5 100644
--- a/src/addon.h
+++ b/src/addon.h
@@ -30,6 +30,7 @@ class Database {
   static Napi::Value InitTokenizer(const Napi::CallbackInfo& info);
   static Napi::Value Close(const Napi::CallbackInfo& info);
   static Napi::Value Exec(const Napi::CallbackInfo& info);
+  static Napi::Value CreateFunction(const Napi::CallbackInfo& info);
 
   fts5_api* GetFTS5API(Napi::Env env);
 
diff --git a/test/memory.test.ts b/test/memory.test.ts
index 4d4e83c..531c251 100644
--- a/test/memory.test.ts
+++ b/test/memory.test.ts
@@ -1,4 +1,4 @@
-import { describe, expect, test, beforeEach, afterEach } from 'vitest';
+import { describe, expect, test, beforeEach, afterEach, vi } from 'vitest';
 
 import Database, { setLogger } from '../lib/index.js';
 
@@ -497,3 +497,51 @@ describe('statement cache', () => {
     );
   });
 });
+
+describe('custom function', () => {
+  let fnDb: Database;
+  let fn: ReturnType<typeof vi.fn>;
+  let bigFn: ReturnType<typeof vi.fn>;
+  beforeEach(() => {
+    fnDb = new Database(':memory:');
+
+    fn = vi.fn();
+    fnDb.createFunction('fn', fn);
+
+    bigFn = vi.fn();
+    fnDb.createFunction('bigFn', bigFn, {
+      bigint: true,
+    });
+  });
+
+  afterEach(() => {
+    fnDb.close();
+  });
+
+  test('it calls the function without args', () => {
+    fnDb.exec(`SELECT fn()`);
+    expect(fn).toHaveBeenCalledWith();
+  });
+
+  test('it calls the function with multiple args', () => {
+    fnDb.exec(`SELECT fn(1, '123', NULL)`);
+    expect(fn).toHaveBeenCalledWith(1, '123', null);
+  });
+
+  test('it calls the function with blob', () => {
+    fnDb.exec(`SELECT fn(x'abba')`);
+    expect(fn).toHaveBeenCalledWith(Buffer.from('abba', 'hex'));
+  });
+
+  test('it uses bigints when configured', () => {
+    fnDb.exec(`SELECT bigFn(123456)`);
+    expect(bigFn).toHaveBeenCalledWith(123456n);
+  });
+
+  test('it throws when function returns a value', () => {
+    fnDb.createFunction('intFn', () => {
+      return 1;
+    });
+    expect(() => fnDb.exec(`SELECT intFn()`)).toThrowError('SQLITE_ERROR');
+  });
+});