Massey, 20, will turn out for the Cougars when not required for first team duty by Terry Matterson and it follows a similar agreement for John Davies with York.
Matterson said: “I feel that this opportunity will benefit Nathan giving him the chance to play against some strong and mature forwards in the Championship. As a prop he will gain more from facing that type of opposition than he is likely to in the Under-20’s competition. And of course we can recall him at short notice if we need to call him back.”
Massey has made two Super League appearances for the Tigers this term and has previously spent time on loan with Featherstone, York and Gateshead. He could make his debut for Keighley this Friday, when the Cougars face Sheffield.
Davies made his debut in their defeat at Doncaster last weekend.