The young hooker, who played for England at Under-16, Under-17 and Under-18 levels, had been a member of the Harlequins setup since the age of 13.
A statement from the club's director of rugby Conor O'Shea read: "Our thoughts and prayers are with Seb's family and friends at this difficult time. Seb was an incredible talent and had already represented England at under age level as well as making his senior debut for Quins at just 20.
"He will be sorely missed by the whole squad as well as by the wider club."
Harlequins have requested that the privacy of Adeniran-Olule's family be respected.