The 30-year-old, who joined the Exiles from Neath in 2009, will, however, continue his career in Wales on a part-time basis.
"It's been a great four years and a great club to play for. I'm really sad to go, but it's a good time for me to finish and just play part-time rugby in Wales, where I'll be much closer to home," Beach told the club's official website.
"I think I'm going to take a step back from playing full-time professional rugby, and it's all about finding a career now. I've got a few options open to me, which I'm looking forward to exploring.
"I hope London Welsh can go on and stay in the Premiership but, if the club is relegated, then I hope they come straight back up again, and we've got the squad and the people behind the scenes to do that. To win the Championship and get promoted was one of the best moments of my career."
Beach was part of the side that won promotion to the Aviva Premiership in 2012.