"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, ahead of today's clash with Harlequins.
"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 (Elwood) has done."
The 44 year old'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.
Harlequins: M Brown; T Williams, M Hopper, T Casson, U Monye; N Evans, D Care; J Marler, J Gray, J Johnston, O Kohn, G Robson, M Fa'asavalu, C Robshaw (capt), N Easter.
Connacht: R Henshaw; T O'Halloran, E Griffin, D McSharry, F Vainikolo; M Jarvis, K Marmion; D Buckley, J Harris-Wright, N White (capt), M Swift, M McCarthy, A Browne, J O'Connor, E McKeon.