The Papua New Guinea international was due to play for the Widnes Vikings in 2018, signing with the English Super League club after representing his country at the World Cup late last year.
His death was confirmed by Widnes.
“We are devastated to learn that Kato Ottio has passed away this afternoon. Kato was an incredibly talented player, with a bright future ahead of him in rugby league,” Vikings chief executive James Rule said.
“This news is all the more tragic, because Kato was due to travel to the UK this week to fulfil his dream of playing first-grade rugby league.
“Having only learned this morning that Kato had developed a sudden health issue whilst training yesterday, we are in absolute shock to now receive this tragic news.”
Ottio never made his first grade NRL debut but was a try-scoring machine for the Canberra Raiders’ feeder team Mounties in the NSW Cup. Former teammates and other rugby league stars in Australia and England took to social media to pass on their condolences.