Titmuss, who rose through the ranks at Manly, reported feeling sick at the club's Narrabeen base and was transported to the Northern Beaches Hospital.
He was then taken to Royal North Shore Hospital where he passed away.
"We are all devastated by this news," Manly coach Des Hasler said in a statement on the club's website.
"Keith was a very popular character amongst the playing group. He will be sorely missed but never forgotten by the Sea Eagles."
Titmuss scored the game-winning try for Manly in the 2017 under-20s grand final and was included in the Sydney-based club's full-time squad for the 2021 NRL season.