diff --git a/Examples/Examples.xcodeproj/project.xcworkspace/xcshareddata/swiftpm/Package.resolved b/Examples/Examples.xcodeproj/project.xcworkspace/xcshareddata/swiftpm/Package.resolved
index c35f7919..bf1fd701 100644
--- a/Examples/Examples.xcodeproj/project.xcworkspace/xcshareddata/swiftpm/Package.resolved
+++ b/Examples/Examples.xcodeproj/project.xcworkspace/xcshareddata/swiftpm/Package.resolved
@@ -123,8 +123,8 @@
       "kind" : "remoteSourceControl",
       "location" : "https://github.com/pointfreeco/swift-structured-queries",
       "state" : {
-        "revision" : "b5b5a9ed9ff321f43a02f07394f2831eba5e11f2",
-        "version" : "0.13.0"
+        "revision" : "e53cd5c87b31eeba77a801eec9a7e82c6d7faaab",
+        "version" : "0.15.0"
       }
     },
     {
diff --git a/Package.resolved b/Package.resolved
index a4e00c4e..a28c3596 100644
--- a/Package.resolved
+++ b/Package.resolved
@@ -1,5 +1,5 @@
 {
-  "originHash" : "d87ac3bfcdef05674d1d54c62277ab77a7f1a74d2b106a3e0a8eb5567e2ff1ed",
+  "originHash" : "96fc55dd49af697ce0e0bf306c46a68e5c589a1cabb04749621327771c159fb5",
   "pins" : [
     {
       "identity" : "combine-schedulers",
@@ -123,8 +123,8 @@
       "kind" : "remoteSourceControl",
       "location" : "https://github.com/pointfreeco/swift-structured-queries",
       "state" : {
-        "revision" : "b5b5a9ed9ff321f43a02f07394f2831eba5e11f2",
-        "version" : "0.13.0"
+        "revision" : "e53cd5c87b31eeba77a801eec9a7e82c6d7faaab",
+        "version" : "0.15.0"
       }
     },
     {
diff --git a/Package.swift b/Package.swift
index d389be9e..41c5f830 100644
--- a/Package.swift
+++ b/Package.swift
@@ -39,7 +39,7 @@ let package = Package(
     .package(url: "https://github.com/pointfreeco/xctest-dynamic-overlay", from: "1.5.0"),
     .package(url: "https://github.com/pointfreeco/swift-sharing", from: "2.3.0"),
     .package(url: "https://github.com/pointfreeco/swift-snapshot-testing", from: "1.18.4"),
-    .package(url: "https://github.com/pointfreeco/swift-structured-queries", from: "0.13.0"),
+    .package(url: "https://github.com/pointfreeco/swift-structured-queries", from: "0.15.0"),
   ],
   targets: [
     .target(
@@ -82,6 +82,7 @@ let package = Package(
         .product(name: "Dependencies", package: "swift-dependencies"),
         .product(name: "IssueReporting", package: "xctest-dynamic-overlay"),
         .product(name: "StructuredQueriesCore", package: "swift-structured-queries"),
+        .product(name: "StructuredQueriesSQLiteCore", package: "swift-structured-queries"),
       ]
     ),
     .target(
@@ -89,6 +90,7 @@ let package = Package(
       dependencies: [
         "StructuredQueriesGRDBCore",
         .product(name: "StructuredQueries", package: "swift-structured-queries"),
+        .product(name: "StructuredQueriesSQLite", package: "swift-structured-queries"),
       ]
     ),
     .testTarget(
diff --git a/Sources/StructuredQueriesGRDB/StructuredQueriesGRDB.swift b/Sources/StructuredQueriesGRDB/StructuredQueriesGRDB.swift
index 99a11a9d..047e9bfe 100644
--- a/Sources/StructuredQueriesGRDB/StructuredQueriesGRDB.swift
+++ b/Sources/StructuredQueriesGRDB/StructuredQueriesGRDB.swift
@@ -1,2 +1,3 @@
 @_exported import StructuredQueries
+@_exported import StructuredQueriesSQLite
 @_exported import StructuredQueriesGRDBCore
diff --git a/Sources/StructuredQueriesGRDBCore/CustomFunctions.swift b/Sources/StructuredQueriesGRDBCore/CustomFunctions.swift
new file mode 100644
index 00000000..2b93a3f2
--- /dev/null
+++ b/Sources/StructuredQueriesGRDBCore/CustomFunctions.swift
@@ -0,0 +1,121 @@
+import Foundation
+import GRDB
+import GRDBSQLite
+
+extension Database {
+  /// Adds a user-defined `@DatabaseFunction` to a connection.
+  ///
+  /// - Parameter function: A database function to add.
+  public func add(function: some ScalarDatabaseFunction) {
+    sqlite3_create_function_v2(
+      sqliteConnection,
+      function.name,
+      function.argumentCount,
+      function.textEncoding,
+      Unmanaged.passRetained(ScalarDatabaseFunctionBox(function)).toOpaque(),
+      { context, argumentCount, arguments in
+        Unmanaged<ScalarDatabaseFunctionBox>
+          .fromOpaque(sqlite3_user_data(context))
+          .takeUnretainedValue()
+          .function
+          .invoke([QueryBinding](argumentCount: argumentCount, arguments: arguments))
+          .result(db: context)
+      },
+      nil,
+      nil,
+      { box in
+        guard let box else { return }
+        Unmanaged<ScalarDatabaseFunctionBox>.fromOpaque(box).release()
+      }
+    )
+  }
+
+  /// Deletes a user-defined `@DatabaseFunction` from a connection.
+  ///
+  /// - Parameter function: A database function to delete.
+  public func remove(function: some ScalarDatabaseFunction) {
+    sqlite3_create_function_v2(
+      sqliteConnection,
+      function.name,
+      function.argumentCount,
+      function.textEncoding,
+      nil,
+      nil,
+      nil,
+      nil,
+      nil
+    )
+  }
+}
+
+extension ScalarDatabaseFunction {
+  fileprivate var argumentCount: Int32 {
+    Int32(argumentCount ?? -1)
+  }
+
+  fileprivate var textEncoding: Int32 {
+    SQLITE_UTF8 | (isDeterministic ? SQLITE_DETERMINISTIC : 0)
+  }
+}
+
+private final class ScalarDatabaseFunctionBox {
+  let function: any ScalarDatabaseFunction
+  init(_ function: some ScalarDatabaseFunction) {
+    self.function = function
+  }
+}
+
+extension [QueryBinding] {
+  fileprivate init(argumentCount: Int32, arguments: UnsafeMutablePointer<OpaquePointer?>?) {
+    self = (0..<argumentCount).map { offset in
+      let value = arguments?[Int(offset)]
+      switch sqlite3_value_type(value) {
+      case SQLITE_BLOB:
+        if let blob = sqlite3_value_blob(value) {
+          let count = Int(sqlite3_value_bytes(value))
+          let buffer = UnsafeRawBufferPointer(start: blob, count: count)
+          return .blob([UInt8](buffer))
+        } else {
+          return .blob([])
+        }
+      case SQLITE_FLOAT:
+        return .double(sqlite3_value_double(value))
+      case SQLITE_INTEGER:
+        return .int(sqlite3_value_int64(value))
+      case SQLITE_NULL:
+        return .null
+      case SQLITE_TEXT:
+        return .text(String(cString: UnsafePointer(sqlite3_value_text(value))))
+      default:
+        return .invalid(UnknownType())
+      }
+    }
+  }
+
+  private struct UnknownType: Error {}
+}
+
+extension QueryBinding {
+  fileprivate func result(db: OpaquePointer?) {
+    switch self {
+    case .blob(let value):
+      sqlite3_result_blob(db, Array(value), Int32(value.count), SQLITE_TRANSIENT)
+    case .double(let value):
+      sqlite3_result_double(db, value)
+    case .date(let value):
+      sqlite3_result_text(db, value.iso8601String, -1, SQLITE_TRANSIENT)
+    case .int(let value):
+      sqlite3_result_int64(db, value)
+    case .null:
+      sqlite3_result_null(db)
+    case .text(let value):
+      sqlite3_result_text(db, value, -1, SQLITE_TRANSIENT)
+    case .uuid(let value):
+      sqlite3_result_text(db, value.uuidString.lowercased(), -1, SQLITE_TRANSIENT)
+    case .invalid(let error):
+      sqlite3_result_error(db, error.underlyingError.localizedDescription, -1)
+    }
+  }
+}
+
+let SQLITE_TRANSIENT = unsafeBitCast(-1, to: sqlite3_destructor_type.self)
diff --git a/Sources/StructuredQueriesGRDBCore/Internal/Exports.swift b/Sources/StructuredQueriesGRDBCore/Internal/Exports.swift
index 320bdbac..e29fce7e 100644
--- a/Sources/StructuredQueriesGRDBCore/Internal/Exports.swift
+++ b/Sources/StructuredQueriesGRDBCore/Internal/Exports.swift
@@ -1 +1,2 @@
 @_exported import StructuredQueriesCore
+@_exported import StructuredQueriesSQLiteCore
diff --git a/Tests/StructuredQueriesGRDBTests/CustomFunctionTests.swift b/Tests/StructuredQueriesGRDBTests/CustomFunctionTests.swift
new file mode 100644
index 00000000..003e3370
--- /dev/null
+++ b/Tests/StructuredQueriesGRDBTests/CustomFunctionTests.swift
@@ -0,0 +1,32 @@
+import Foundation
+import GRDB
+import StructuredQueriesGRDB
+import Testing
+
+@Suite struct CustomFunctionsTests {
+  @DatabaseFunction func customDate() -> Date {
+    Date(timeIntervalSinceReferenceDate: 0)
+  }
+
+  @Test func basics() throws {
+    var configuration = Configuration()
+    configuration.prepareDatabase { db in
+      db.add(function: $customDate)
+    }
+    let database = try DatabaseQueue(configuration: configuration)
+    let date = try database.read { db in
+      try Values($customDate())
+        .fetchOne(db)
+    }
+    #expect(date?.timeIntervalSinceReferenceDate == 0)
+
+    try database.write { db in
+      db.remove(function: $customDate)
+    }
+    #expect(throws: (any Error).self) {
+      try database.read { db in
+        _ = try Values($customDate()).fetchOne(db)
+      }
+    }
+  }
+}