The popular tighthead prop has made more than 200 appearances for Bath since arriving from Pontypridd in 2003.
Bell went on to make five appearances for England, while he won the Churchill Cup with the Saxons in 2010.
"I have had a fantastic career, from starting out at Quins through to the last nine years here with Bath," said Bell. "I began playing rugby for Bath minis, so to have had such a long career here has been amazing and it's been an honour to run out in the Bath shirt over 200 times.
"I'd like to thank all the players, coaches and staff I've worked with over the years, and also the supporters who supported me throughout my career.
"It's been a difficult decision to make, but I'm looking forward to the new challenges that await, primarily with my business Bell Financial Services."
Bell revealed that he had been battling against depression for long periods of his career.
And the RPA have praised Bell for his decision to talk about his illness in a bid to help others.
RPA chief executive Damian Hopley said: "On behalf of all of our RPA members I would like to commend and congratulate Duncan Bell, not only on his outstanding rugby career, but also on his extremely brave decision to address the issue of depression as he retires from the game in order to help his peers who may be facing similar challenges in their lives.
"Duncan is widely respected as one of the most talented and enduring players in the Bath squad, with excellent footballing skills that deservedly earned him full England honours, and his humble and approachable nature have made him one of the most popular players ever to have played for club or country.
"To the outside world, Duncan has always appeared the most upbeat of characters, rarely without a smile on his face and always one to make or take a joke. Privately, Duncan has had some considerable challenges in his life and I cannot speak highly enough of the courage he is now showing to speak openly about the depression he has suffered from over the past few years.
"We know how uncomfortable Duncan has found it to openly discuss this illness, which until recently has been seen as a taboo subject in professional sport and it is to his tremendous credit he is confronting this head on. The fact that someone as popular and as well-known as Duncan is coming forward to speak about his personal struggle will help to raise awareness of the issue and will reassure others who may be suffering in silence.
"On behalf of everyone at The RPA, I would like to thank Duncan for having put his own anxieties and concerns to one side to help us produce a video for our Mental Health seminars which are being presented to players at every Aviva Premiership club from the end of this season."
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