Moreira is joining the Swans, pending a successful medical.
"I believe the experience I have gained at such a big club as Benfica and with Portugal will help me achieve success in the Premier League with Swansea," he said.
"I've had very good conversations with the manager and the chairman (Huw Jenkins) and had a good look around Swansea and the stadium, which is beautiful.
"I watched Swansea's game at Wembley against Reading on the internet and I was very impressed. I can't wait to get started."