Catt, who helped England win the World Cup in 2003, has opted to hang up his boots in order to focus on his coaching career with the Exiles.
The 38-year-old won 75 England caps, played in the 2003 and 2007 World Cup finals and also featured for the British and Irish Lions.
"I've been fortunate to have had an enjoyable and successful career as a player at club and international level," Catt said.
"London Irish has given me the opportunity to make the transition from playing to coaching, and the time is right for me to focus full-time on that role.
"I hope to help the club maintain the progress it has made in recent seasons.
"I would like to thank all the players, coaches, sponsors, supporters and my family who have encouraged me throughout my playing career."
Irish head coach Toby Booth added: "He is an inspirational sportsman, and we look forward to working with him as he continues the transition into what we are sure will be a very successful coaching career with us."
Catt, who was born in South Africa, made his England debut in 1994 and was part of their Five Nations title-winning Grand Slam season the following year.
He played in England's World Cup final win against Australia in Sydney and retired from international rugby after the 2007 World Cup final defeat against South Africa.
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