With Ireland and Lions full-back Geordan Murphy a long-time Tiger, Cullen, his former Leinster colleague Shane Jennings and Belfast Harlequins fly-half Ian Humphreys have all completed terms with the two-time Heineken Cup champions in the last two months.
The 27-year-old Cullen, capped 15 times for Ireland, sees the move as stepping stone to furthering his Test chances ahead of next year's Six Nations and the impending 2007 World Cup.
"Since making my senior Leinster debut in 1998, I have won more than 80 caps for my province and been lucky and privileged enough to captain the team on a number of occasions. I have had seven very happy years in the blue of Leinster and until I was contacted by Leicester Tigers last week, I had not given any serious thought to leaving," admitted the Blackrock College giant.
"However, when a club like Leicester makes you an offer, I feel it's something you cannot ignore. Tigers are one of the top sides in the Premiership, they have won the Heineken Cup twice in recent seasons and they have a squad that every player would love to be part of.
New Tigers coach Pat Howard told the club's website: "Leo brings second row experience and plenty of captaincy experience, which will be important with the Lions guys away at the start of the season.
"Leo's signing complements what we already have at the club and he will help to bring on the likes of James Hamilton and Ian Nimmo during the new season."
Cullen lined out 23 times, including all seven Heineken Cup ties, for Leinster in 2004/05 - his seventh straight season.