The Dutch midfielder made just 13 appearances last season in the Premier League due to injuries but has extended his stay at the Liberty Stadium until June 2014
Agustien would have been out of contract next month but has put pen to paper on a deal which will keep him at the Liberty Stadium until June 2014,
The Dutchman suffered with injuries last season, making just 13 appearances in Swansea’s first Premier League campaign, including six from the bench.
However, the 25-year-old now hopes to be firing on all cylinders for the new season after missing the last two months of the previous campaign.
"I've already started working towards next season," said Agustien. "I want to return fit and fresh.
"I'm delighted to sign the new deal because it's a great time to be part of Swansea City and I want to help the club go higher and higher.
"The team has done extremely well this season, but we will only get better and I want to be part of that.
"Playing for the club on the Premier League stage is where I want to be because I've worked hard to make that dream become a reality.”
Agustien also revealed that he is still recovering from his injury but has thanked the fans for the support they have shown him this season.
"I've still got a bit of rehab to do on my ankle before I start running properly, but I'm working hard in the gym at the moment,” he added.
"While it's been a frustrating season at times with the injuries, I'd like to think that I have shown people what I'm capable of during the games I have played.
“I always try to be positive and do my best for the team and the fans because they've been great with me.''