The 29-year-old represented Great Britain at three Olympic Games, also winning two bronze medals between 2008 and 2016.
He won three World Championship silver medals, two European golds and the 2006 Commonwealth Games pommel horse title.
“I had to make the difficult decision to hang up my leotard,” said Smith, who in December 2012 was appointed MBE for services to gymnastics.
After the London Games, where he also earned team bronze, Smith took a lengthy break from competition and training, only returning in time to earn his place in the British team for Rio. Until this week, no-one was quite sure if he might pull the same trick again, but this showbusiness opportunity has clearly made up his mind. He turned down a choral scholarship as a youngster because of his passion for gymnastics, but is now finally turning to the stage full-time.
Smith, who was diagnosed with ADHD at the age of seven, says he had intended to return to gymnastics training for the Tokyo 2020 Olympics, but changed his mind due to changes in qualification criteria and new “exciting opportunities”.
“Gym has given me everything,” said Smith, who announced his retirement on ITV show Lorraine. “It gave me the tools necessary to channel my ADHD throughout my childhood, and as I grew up through the sport, I 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.”
Gymnasts divide into all-rounders and specialists, and Smith was definitely a specialist. Like many in this sport, he started out as a hyperactive child who needed an outlet for his excessive energy. He also suffered from attention-deficit disorder, which doctors treated with heavy doses of Ritalin.
His bronze medal at the 2008 Beijing Games. saw him win Britains first Olympic gymnastics medal since 1928 and become the first British man since 1908 to win an individual medal.
Smith, who is to star in musical Rip it Up on Londons West End in February, won series the BBC show Strictly Come Dancing in 2012.
His career has not been without controversy – in November 2016 he was given a two-month ban by British Gymnastics after appearing to mock Islam in a video. Smith said he was “deeply sorry” for his “thoughtless actions”.
Former GB team-mate Dan Keatings paid tribute to Smith, saying: “You set the benchmark and made British gymnasts believe we can mix it with the best in the world.
“British gymnastics wouldnt be where it is now without you. Youve achieved your dream winning multiple medals in all major competitions and can leave the sport with your head held high.”
Smith was Great Britains first individual Olympic medallist in gymnastics for a century when he won bronze on the pommel horse at the 2008 Games in Beijing.
He won two medals at London 2012, silver in the pommel and a team bronze, and secured another individual silver in Rio four years later.
Smith wrote on Facebook: 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.
Read more Im so proud of what Ive achieved in the sport and hope that my achievements have left a legacy that will continue to inspire gymnasts of the future.
Smith had been working towards qualification for the Tokyo Olympics in 2020, but now says he will pursue other exciting opportunities that Ive been presented with.
Smith, who also won three World Championship medals over the course of his career, was the winner of the BBCs Strictly Come Dancing programme in 2012.
He was given a two-month ban by British Gymnastics in 2016 after mocking Islam in a video. Smith apologised for the footage, which showed him pretending to pray to Allah while laughing, and revealed he had received a number of death threats.