The former Tigers, England and Lions prop will join Ian McGeechan’s backroom team as scrum coach, where he will work alongside Warren Gatland (forwards), Shaun Edwards (defence) and Rob Howley (backs).
Rowntree, 37, made 398 appearances for Tigers between 1990 and 2007, won 54 England caps and toured South Africa in 1997 and New Zealand in 2005 with the Lions.
He said: “It is a tremendous honour to be asked to become involved with Geech’s coaching set up.
“Having experienced both the highs and lows of Lions rugby as a player, I am determined to be part of a winning set up in 2009 and am relishing the opportunity to work with the other coaches to make that happen.
“Geech is someone that I have an enormous amount of respect for and I cannot wait to play my part in contributing to a great rugby environment.”
Lions head coach Ian McGeechan said: “I am delighted to have Graham on board. He brings with him a direct approach and an understanding of not only what we are looking to achieve as a coaching group but also an understanding of the Lions ethos.
“The scrum will be an intensely contested area in South Africa and we’ll need to be technically accurate at scrum time from the moment we arrive. It is crucial to have a forward pack that are technically as good as they can be against a renowned set of South African forwards.”
Martin Johnson, England team manager and Tigers great, said: “On behalf of the England squad I'm delighted for Graham and it's a well deserved honour.
“Having played alongside Graham for many years at club and international level and having seen him coach the England squad I know what an asset he will be to the British & Irish Lions who he served so well as a squad member in 1997 and 2005.
“I will give due consideration to replacing Graham for the tour to Argentina.”
