Moody joined Leicester from Oakham School and has made 216 first-team appearances since his debut in August 1996, scoring 34 tries.
Awarded an MBE following England’s Rugby World Cup win in 2003, he has gained 59 senior caps and played in three Tests with the British & Irish Lions.
Leicester Tigers chairman Peter Tom CBE: “We have all seen Lewis grow from an Oakham schoolboy into an established international player during 14 years with Tigers’ first-team squad.
“He has played in some of our most successful teams in the club’s history and we will all wish him well when it comes to the end of the season and he moves on to a fresh challenge.”
Leicester Tigers head coach Richard Cockerill said: “Lewis has been a big part of this club for a long time and he has won every major honour in his time here.
“Everyone knows about the enthusiasm he brings to his game, and that has not altered during his 14 years in the first-team squad, which says a lot for his attitude to the game of rugby.
“We wish him well as he takes up a new challenge for next season and I know he will receive a very warm welcome from our supporters whenever he returns to Welford Road.
“I’m sure he will be looking to go out on a high note and we look forward to his return to Leicester for the run-in to the end of this season after international duty at this season’s Six Nations.”
Bath announced the signing on Friday lunchtime and Moody said: "I am really looking forward to an exciting new challenge at this stage of my career."
Bath's head coach Steve Meehan added: "Lewis is a proven winner and he brings vast experience, a fierce determination and great attitude with him."