The 23-year-old has made 85 appearances for the Aviva Premiership side since 2011 and has also featured for England at under-18, under-19 and under-20 levels.
John Kingston, Harlequins' director of rugby, said: "I am thrilled that Kyle has signed a long term contract to remain at Harlequins. He is another successful product of our fantastic Academy and as such, his recent significant development is even more satisfying.
"I have no doubt that he has the necessary qualities to be a major influence in our quest for further trophies over the coming period."
Sinckler added: "Renewing my contract with Harlequins means a massive amount to me. The club has done so much for me over the years and I'm continuously wanting to repay the faith it has put in me.
"This group of players has a great amount of potential and we need to be pushing for top four and silverware again. I'm looking forward to helping to create history with this great club."
Harlequins are currently 11th in the nascent Premiership table.