According to his club the Manly Warringah Sea Eagles, Titmuss was taken ill after a training session in the Sydney suburb of Narrabeen, Australia.
He was taken by ambulance to Northern Beaches Hospital and later transferred to Royal North Shore Hospital, where he died shortly after.
“We are all devastated by this news,” said Sea Eagles coach Des Hasler.
“Keith was a very popular character amongst the playing group. He will be sorely missed but never forgotten by the Sea Eagles.”
Titmuss, a prop forward who played for the Sea Eagles’ junior teams before graduating to the club’s National Rugby League (NRL) squad, scored the Sea Eagles’ winning try in the National Youth Competition (NYC) Grand Final in 2017. He also won the club’s players’ player award in last year’s Jersey Flegg Cup.
“This is a tragic day for rugby league,” said NRL CEO Andrew Abdo.
“The game has lost a promising young player with the world at his feet. Keith had made it into Manly’s Top 30 for the 2021 season after coming through the club’s junior ranks.
“We will make sure Keith’s family, the club and our players receive every support they need from the game.
“We are one family and today we have lost a member of that family. On behalf of the commission and the game I send my deepest condolences to Keith’s family.”
According to Abdo, NRL wellbeing staff are offering support to the Sea Eagles’ players and staff, which includes grief counselling.
“This is devastating news. Love and thoughts to his family and friends, absolutely tragic,” Wade Graham, who has represented Australia at rugby league, wrote on social media. “Thinking of the Sea Eagles family at the moment.”
“Heartbreaking,” Wigan Warriors and former Sea Eagles player Jackson Hastings wrote on Twitter. “Praying for Keith’s family, friends & teammates. Sending love! RIP.”