The 21-year-old has spent time on loan with York this season and Millers boss Steve Evans is already looking to make his stay at New York a permanent one.
Evans told Rotherham's official website: "As a manager I'm delighted that, after a number of attempts, I have secured the signature of young Daniel Kearns to join the Millers, initially on loan until of the season, with a view to making it a permanent transfer in the summer.
"Back in 2011 I tried to convince the boy to sign for my previous club (Crawley), but, at that time, he decided to sign for Peterborough United.
"He has developed under Darren Ferguson and the Republic of Ireland Under 21 international is a real capture for us.
"Sadly, I am not at liberty to reveal the League One and League Two clubs who have sought his signature in the last two weeks."
Kearns could make his debut against Barnet this Saturday when Edgar Davids' men make the trip to South Yorkshire.
Evans added: "Our supporters will enjoy watching this young man; his ability to take on defenders and deliver a telling pass will become obvious to all.
"I would like to place on record my thanks to Darren Ferguson for the very positive comments he gave about myself and the club to young Daniel."