A former Ireland Schools international, Cronin featured in the All-Ireland League for Shannon and he earned his first Ireland cap against Fiji at the RDS in November 2009, incidentally in the same match as Jonathan Sexton and Sean O'Brien.
He made his first Test start against New Zealand in the 2010 summer tour and also started against Australia. Cronin has also represented Ireland Schools, Under-19, Under-21 and 'A' (Ireland Wolfhounds) levels.
The Limerick man, aged 24, played three games in the recent GUINNESS series against Samoa, New Zealand and Argentina and has featured seven times for Ireland to date.
Cronin said: "I am delighted to put pen to paper on a deal to join Leinster this summer and I believe that with the exciting players in the squad, the future looks bright.
"I have been hugely impressed by Leinster's displays both in the Magners League and in the Heineken Cup so far this season and the province's ambitions mirror my own.
"Hopefully, I can establish myself in the squad when I join up in a few months' time."
He added: "I am grateful for the opportunity that Connacht have given me over the last three years to play for a fantastic side.
"They will continue to grow from strength to strength under Eric Elwood and the existing management team and I will do my utmost to help improve our Magners League standing over the coming months."
Commenting on Cronin's signing, Leinster head coach Joe Schmidt said: "It's great that Sean has confirmed that he will join us this summer.
"He is a dynamic and talented front rower who will add experience at hooker, following the retirement of John Fogarty earlier this season."