Standard Sport reported on Wednesday the teenager’s agreement to move to the Yorkshire club after Hull beat AFC Wimbledon to his services.
Simons made his debut in the Carabao Cup last season before agreeing a new contract at Stamford Bridge, which features a club option to extend his stay into 2024.
Norwich, Watford, Southampton and Brighton were all keen on signing the versatile defensive midfielder for free as he neared the end of his previous deal.
Simons becomes the 16th player to leave Chelsea on loan this summer.
Romelu Lukaku, Callum Hudson-Odoi and Levi Colwill are among those to have also left on a temporary basis with Baba Rahmann re-signing for Reading on Wednesday.
Chelsea pulled out of a move for Dynamo Moscow midfielder Arsen Zakharyan on transfer deadline day as they looked to seal deals for Pierre-Emerick Aubameyang and Edson Alvarez.