Head coach Leo Cullen hailed the coup, backing Leinster to gain enormous benefit from Henry's expertise.
Henry will join Leinster on July 30 and remain at the Dublin area outfit for several weeks in the build-up to the 2016/17 season.
"This isn't by any means a first for Leinster Rugby and I felt it was important for myself and the wider coaching and playing staff in Leinster to look at different ways of challenging ourselves," said Leinster boss Cullen.
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"To that end I am delighted that Graham has decided to work with us and I would like to publicly thank the Professional Games Board in Leinster for their support with this decision.
"Everyone in the organisation will benefit from his expertise and undoubtedly the coaches and the players will benefit from having a coach of his stature in and amongst us as we build towards the new season."
Henry guided Auckland Blues to two Super Rugby titles before taking the helm with Wales in 1998, progressing to coach the British and Irish Lions in Australia in 2001.
After stepping up to lead New Zealand in 2004 Henry sealed the All Blacks' first World Cup win since 1987 on home soil in 2011.
"It's a privilege to be asked to assist Leo and his team at Leinster, a club with such a rich and proud history," said Henry.
