The New Zealander, a former coach of the Fiji national side, won the Guinness Pro12 trophy with the Welsh side Scarlets in 2017.
"In Wayne Pivac, we have secured the best man for the job and we have done so rigorously and decisively to the collective benefit of all involved in Welsh rugby," Welsh Rugby Union chief executive Martyn Phillips said at a news conference.
Pivac, who has been coaching for more than 20 years with spells with Takapuna, North Harbour, Northland and Auckland, said he was relishing the challenge.
