The 26-year-old, who represents Plymouth Leander, has won World, European and Commonwealth titles but is just missing an Olympic medal from his impressive collection.
He was fourth in 50m freestyle at the Rio Games in 2016 and on Sunday morning finished fifth in the same event in Tokyo.
In a tight final, he was just 0.17 of a second off winning silver in an event where American Caeleb Dressel dominated.
Dressel swam an Olympic record of 21.07 but no-one else went under 21.55. Proud, who does have a lifetime best of 21.11, clocked 21.72 seconds swimming in lane two.
“It’s a tough one, that’s for sure,” said a disappointed Proud afterwards. “A lot of people have invested their time and energy into me and I wish I could’ve brought them something home, but that’s just how it is.”
The former Plymouth College pupil added: “Swimming has taken me on an amazing journey. It’s been a tough year – a tough couple of years – but I’m still motivated to come back and keep swimming. My family, my girlfriend, everyone has been massively supportive.
“The good thing is some of the athletes in there are getting well into their 30s and are still going, so I’ve got a couple more years left. I thought this one was a good one for me, it just didn’t quite happen but that’s how it is.”
Meanwhile, former Mount Kelly pupil Federico Burdisso won a second Olympic medal in Tokyo.
The 19-year-old, who had won an individual bronze in the men’s 200m butterfly final, added another bronze with the Italy men’s medley relay team, who came third behind the USA and Great Britain.