Edinburgh's 'Monster' won the belt by beating Simon Barclay in October last year but has opted to hang up his gloves in the wake of March's British title defeat to Matty Askin.
Simmons (18-3-KO7) said: "It's time to enjoy spending more time with my wife and baby. I'm happy and content with retiring at this stage of my career.
Matty Askin defeats Stephen Simmons in second-round
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Stephen Simmons: Beating an Englishman in his own back yard is even more special
Simmons turned professional in 2011 and won his first 11 contests. In 2016, he faced Noel Gevor for the WBC International cruiserweight title and was only denied a step up to world level by a controversial points defeat.