Lam, who has accepted a two-year contract with the option to extend for a further period, represented the All Blacks in a tour match before going on to captain Samoa in a distinguished playing career which included spells with Newcastle Falcons and the Saints.
"My family and I are truly blessed to have another opportunity to be part of a new community and country and we are looking forward to the exciting challenge ahead at Connacht," said Lam.
"I was very impressed by the Board's vision for Connacht Rugby and the fighting spirit of the team and supporters to persevere against all the odds. I'm looking forward to working with good people to build on the great work that Eric has done."
Lam's coaching career includes a successful spell at Auckland in the New Zealand Provincial Championship from 2005-9 where he won the title in 2005 and 2007 before he went on to head up the Auckland Blues in the Super 15.
Under Lam the Blues achieved their best finish for several years in 2011 reaching the semi-finals, where they lost to eventual winners the Queensland Reds. He has also been involved at international level as a consultant to both Scotland and Samoa, most recently on the Pacific Islanders highly successful UK tour late last year when they claimed the scalps of both Wales and Canada which saw them make the top eight of the IRB World Rankings.
Connacht Rugby Chief Executive Tom Sears said: "The appointment of our next coach is a huge decision for Connacht Rugby and at the end of an extensive process we are delighted to be able to appoint Pat Lam.
"The position attracted worldwide interest enabling us to select from an incredibly strong field and we are convinced Pat has all the qualities and credentials to take Connacht forward.
"Allied to his experience and technical expertise he displayed an excellent knowledge of Connacht, of where we are, what we aim to achieve, the obstacles we face and how we can ultimately achieve our objective of becoming one of the leading sides in European rugby.
"Throughout the selection process Pat displayed a real affinity to what Connacht Rugby means and that was crucial to us. He will no doubt bring about progressive changes but he is also extremely committed to retaining and building on the qualities that make Connacht unique."