The midfielder leaves hometown club Newcastle United having come right through the club's academy system.
Longstaff has signed a deal to 2029 at Elland Road.
He told the Leeds website: “I remember obviously playing here as a little kid, nine, 10 years old. It is funny where life takes you. So to be back here now with the plans for the club and stuff, I think it is just really exciting to get going.
“I spoke to (Karl) Darlow straight away, but I have got to meet everyone. It seems like a really good group. I have heard great things.
“I think there is something to be said for a team that wants you and a club that really wants you. I think from the first time I spoke to Leeds, I sort of got a feeling that they really wanted me.
“It just fills you full of confidence and it just makes you want to repay those people. It is a massive, massive football club similar to Newcastle in a lot of ways.
“It was going to take a lot for us to leave Newcastle to be honest and I wasn’t just going to leave to go anywhere so it had to be the right place for me and my family and I think this is a perfect place.
“I got to watch pretty much all the games and see a team that was really exciting to watch, athletic, really good on the ball.
“Every time I came to play at Leeds the atmosphere was amazing it was always bouncing and a really tough place to play and just looking forward to having that on my side rather than having to play against.”