Keller, who played over 100 games for the U.S., enjoyed an extended career in Europe before joining Seattle in 2009. Keller spent the last three seasons of his career with the Sounders.
The 41-year-old retired at the end of the 2011 season, ending his 20-year pro career. An all-star in each of his three seasons in MLS, Keller won his first MLS Goalkeeper of the Year award.
"It was a great year and I'm proud that I was able to perform at the level in my final year that I hoped to achieve," said Keller. "Anytime you receive an award it's always nice that someone has thought of you in that regard.
"I have never played the game for the awards but it's nice to be recognized by your peers."
Keller played in every minute of every match for Sounders FC in 2011, and led the team to the second-best record in MLS. He won a league-high 18 games this season and led all starting goalkeepers in saves percentage (76 percent) while posting a 1.09 goals against average and nine shutouts.
"Kasey had a superb season, finishing his career at such a high level is a reflection of his professionalism and high standards," said Sounders FC coach Sigi Schmid. "The game will miss him."
Philadelphia Union goalie Faryd Mondragon finished second in the voting, while FC Dallas goalie Kevin Hartman was third. The award was voted on by players, club representatives and media.