Rowntree, who won 54 England caps and is currently Harlequins' forwards coach, will serve as Gatland's assistant as the Lions attempt to win their first series against the All Blacks since 1971.
Rowntree will continue in his role at the Stoop following the Tour's conclusion and said: “I am honoured to be asked to tour with the British and Irish Lions again and I really am thankful to Harlequins, John Kingston and David Ellis for allowing me to go.
“Having experienced the highs and the lows of the Lions as a player and a coach, I know how special it is.
"This year will be the ultimate challenge considering the form that New Zealand have been in across the last decade.
"But looking at the talent and potential in England, Ireland, Scotland and Wales, it is a hugely exciting opportunity for us to do something incredible.
“I am grateful and excited for the opportunity ahead, but my attention will firmly remain on Harlequins until the conclusion of our season," he added.
Neil Jenkins has also been confirmed as a member of Gatland's coaching team, while Steve Borthwick, Andy Farrell and Rob Howley also feature.
The Lions are set to jet off in June and Gatland is confident he has the right team in place to deliver success.
Gatland said: “We all know what Neil and Graham bring – a huge amount of knowledge, experience and expertise at club, international and eight Lions Tours between them.
"Given the schedule, with two games a week, having six coaches will allow us to devote enough time to ensuring we are successful.
“I believe we have a coaching team that will get the best out of what will be a highly talented and motivated group of players capable of winning the Test series.”