The 21-year old defender has penned a two-year extension which will keep him at the Liberty Stadium until 2016.
The Wales U21 international has spent time on loan with Wrexham this season and is now looking forward to furthering his development with the Swans.
"I'm absolutely delighted," Alfei said. "It's good to know the club has faith in me and there is nowhere I would rather be than Swansea.
"The loan spell at Wrexham was great for me. It's a good club and I really believe I learnt a lot there this season. It's made me a better player.
"Obviously it's been difficult at times for me this season because of the injury, especially after missing out on their two trips to Wembley (FA Trophy final and play-off final) - what are the odds of that happening?
"But that's all part of my development and I wouldn't rule out going on loan somewhere again next season. I think that's probably what I need to keep progressing before hopefully returning to push hard for a place in the first team."