The Papua New Guinean player was scheduled to fly to the UK this week to begin preparations for the new Super League season, which begins in February.
A statement from Widnes chief executive James Rule confirming the tragic news read: "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. 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.
"We had been in regular contact with Kato and were excited to welcome a bright, excited and passionate young man, who had genuine potential for the future. 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.
"We will be liaising closely with his loved ones to offer them our condolences and support at this difficult time. Widnes Vikings will of course be looking to celebrate Kato's life and pay our sincere respects to someone who we believe would have become a hero at our club. Whilst we are coming to terms with this news, and still learning the full facts of this situation, we are unable to comment further at this time."
Head coach Denis Betts added: "To receive this tragic news is heart-breaking. All of us saw at the World Cup the incredible gifts that Kato had, and his potential to forge a great career in Rugby League. Moreover, in all of our interactions with him, we found Kato to be a person of great character, who was excited to be following his dreams. On behalf of the coaching team, staff and players at Widnes Vikings, I would like to offer our sincere condolences to Kato's loved ones."
Ottio played against England in the quarter-finals of the World Cup in November.