To, who was born in Hong Kong and raised in Australia, died on Monday. The cause of death was not disclosed.
"He felt unwell at a training session and was taken to hospital where sadly, he passed away," the Hong Kong Sports Institute, where To was an Elite Scholarship Athlete since 2016, said in a statement.
"He was extremely popular and loved by his team mates and competitors. Kenneth was known as a truly exceptional person, warm, funny and kind. His sudden passing is a huge loss to local sports."
To, who transferred his nationality in 2016 to swim for Hong Kong, first began to shine at the 2010 Youth Olympic Games in Singapore where he earned six medals, including gold in the 4x100m medley.
In 2012, To won the men's overall title in the FINA Swimming World Cup, before claiming three medals in the FINA World Swimming Championships in Istanbul.
He was also a member of the Australian teams at the 2014 Commonwealth Games that captured gold in the 4x100m freestyle and silver in the 4x100m medley.
World swimming governing body FINA said in a statement it learned with "immense sadness" about To's death and described him as a reference and example for the youth in the territory.
Australian swimmer James Magnussen posted a tribute to To on Instagram https://www.instagram.com/james.magnussen/?hl=en saying: "Shocking news to hear of the passing of my competitor but most of all, friend - @kennethkhto. We've been racing each other and making teams together since we were 16.
"He will remain one of the fiercest competitors I've ever stood on the blocks next to. I firmly believe that for a long period he was pound for pound, the best swimmer in the world.
"RIP little guy, you won't be forgotten."
(Reporting by Frank Pingue in Toronto; Editing by Christian Radnedge)
