The 33-year-old, who has made 138 appearances for Connacht since joining from London Irish in 2006, has admitted that he is "happy" to step away from the sport.
"While I will miss playing rugby immensely, I'm happy to retire at this stage of my life. I've had a great 16 years in the professional game and I'm proud that I've played with two fantastic clubs within that period," he said in a statement.
"I am especially proud to have been involved with Connacht Rugby over the last number of years as the club has seen huge improvements.
"I'm honoured to have been associated with that change and hopefully I have contributed as much as I could. Galway has become a very special place for me, especially with my two sons Samuel and Joseph being born here.
"With a degree in Sport Science & Management Studies as well as a Level 2 IRFU Coaching qualification, I am ready for the next step in my career and I look forward to new opportunities within rugby."
Flavin was capped at youth level for Ireland.