The four-times Olympic medallist revealed the news on his Facebook page after 25 years in the sport.
Peterborough-born Smith claimed a silver medal on the pommel horse at the Rio Olympics in 2016 and had vowed to continue his search for an elusive World or Olympic gold medal.
Smith also claimed bronze in the pommel horse at Beijing 2008, bronze in the team event at London 2012 and another silver in the pommel horse in London.
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But he feels the opportunities he has been offered outside of gymnastics are too good to turn down.
"The day that every sportsman and woman knows will come at some point has finally arrived for me, as today I’m announcing my retirement from competitive gymnastics after 25 years in the sport," said Smith, who won gold on the pommel horse at the 2006 Commonwealth Games in Melbourne.
"Gym has given me everything - it gave me the tools necessary to channel my ADHD throughout my childhood , and as I grew up through the sport, I very quickly found a purpose in life, and it has made me the person I am today.
"From a young age it became my dream to compete at an Olympic Games and represent Great Britain, so to have had the opportunity to do that on three separate occasions, and bring home four Olympic medals in the process, is something I will forever cherish and for which I am eternally grateful.
"I’m so proud of what I’ve achieved in the sport and hope that my achievements have left a legacy that will continue to inspire gymnasts of the future. In addition to the continued development of my gymnastic academies, I have every intention of giving back as much as I can to a sport that gave me so much," said Smith, who won the BBC's Strictly Come Dancing Series after the London Olympics in 2012.
"For the past 18 months my intention had always been to return to full training this month, ahead of competing in the upcoming World Cup series of competitions, so as to hopefully qualify my place at the Tokyo Olympics.
"Recently, with the changes in the qualification criteria, in addition to exciting opportunities that I’ve been presented with as I look to the next stage of my career beyond gymnastics, I had to make the very difficult decision to finally hang up my leotard."
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