The 25-year-old, who joined the Rhinos last year from Salford Red Devils, agreed a new five-year contract to tie him to the Yorkshire outfit until the end of the 2018 season.
"This is fantastic news for the club and our plans for the next few years," McDermott told the club's official website following the announcement. "Joel is pretty unique in the modern game given that he came from the National Rugby League at a young age and has committed his future to the Rhinos and our competition.
"His try ratio is better than one in every two games and he has come up with some big tries for us, including last Saturday's effort against St Helens.
"I think it speaks volumes about how committed the players are to one another that so many are happy to agree long-term contracts with the club and see their future with the Rhinos."
Leeds Rhinos are currently top of the Super League table.