He captained Ireland to the Under-21 Triple Crown in 1998/'99 and represented Ireland at Schools (7 caps), Under-19 (4 caps), Under-21 (16 caps), 'A' (15 caps) and Senior level (18 caps) since his debut in 2002 against New Zealand.
The Wicklow native made 83 appearances for Leinster (including 36 in the Heineken Cup) since making his debut in 1998 against Edinburgh.
Since moving to England from Leinster in 2005, Cullen - 28 - has made 45 appearances for the Tigers, including 10 as captain.
Cullen is the second Tiger to re-join Leinster following from the recent signing of Shane Jennings and is the fourth to commit to the province in recent weeks following that of captain Brian O'Driscoll and Felipe Contepomi.
Leinster Chief Executive Michael Dawson said: "We are delighted to announce Leo's return to Leinster following a hugely successful spell in England.
"Leo has a wealth of experience at both club and international level and will add a great deal of competition in the pack for next season and beyond."
Leo Cullen commented: "Leaving Leicester was a difficult decision, and I had to think long and hard about it. I have really enjoyed my time with the Tigers and I feel that I'm a better player for the experience. The training environment is second to none and it has been great to have been part of such a prestigious club.
"We are still well placed in all three competitions and it would cap off two great seasons personally if I could help play a part in winning some much sought after silverware. I'd like to thank all my Leicester colleagues, especially my fellow players, and all the coaching, management, fitness and medical teams.
"What I will remember most about my time with Leicester is the fans - they are just outstanding the way they get behind the team. You couldn't ask for better supporters, and I will always think very highly of them. Playing in Welford Road is a special experience and to lead the team out that I have admired for so many years on a few occasions was a real privilege.
"Now that I am committed to returning to Leinster, I am very excited about the future. These are good times for the province, there is a great coaching set-up in place and a young and very talented group of players. I want to be part of that again and it would mean a lot to me to win a major trophy with Leinster. I am sorry to be saying goodbye to Leicester Tigers but when I got the opportunity to come home to Leinster, I knew the time had come to do so. I am a Leinsterman born and bred, my family and friends are all here, and this is where I want to be."
