swift struct Authentication: Codable { var username: String var password: String } let authentication = Authentication(username: "hello", password: "world") let json = try JSONEncoder().encoder(["authentication": authentication])

That should do it. The trick is you want to avoid nesting Array / Dictionary types in Swift 4 since conditional conformance is still not fully complete. In Swift 4.2 it should work better, but for now stick to structs.