Kelly joins Rovers on a season-long loan.
“I’ve been looking to get out on loan for a while now and I think that I’ve found the right club so I’m really happy to be here,” Kelly said.
“The gaffer spoke with me and my agent while I was out in America with West Ham last week and everything he said was appealing to me about the style of play and where he sees me playing.
“There was the club as a whole and just a lot of reasons why I chose Doncaster.”
Rovers boss Grant McCann said: “We’re delighted to get PK. He’s a lad we’ve watched a lot of and we had to be wait patiently as he’s been with the West Ham first team all of pre-season.
“He’s capable of playing in the six or eight in our midfield - very much in the mould of Matty Craig. He’s flourished at U21 level with Northern Ireland and has been drafted into training camps with the senior team.
“What we like is that he’s excellent on the ball and an all-action type that can jump and press and recover with ease. He can also beat people which is very important for us.
“He’s been a part of a very successful West Ham youth set up and will add some serious competition to our midfield.
“Huge thanks to Mark Noble, Steve Potts and Kenny Brown for trusting us in PK’s development.”