The club has also revealed that the deal includes an option for a further year's stay at the Liberty Stadium.
"Since the first day I joined this club, I knew something special was happening," Tremmel told Swansea's official website.
"I feel very, very good here. Of course, last season I didn't play the amount of games I had hoped for, but this season has been a completely different story on the pitch for me.
"The club has been on a remarkable rise over the last few years, and it really is a nice story – I was keen to be a part of that story, so the new contract is great news for me because I feel I have a lot more to give this club."
Swansea are currently eighth in the Premier League.