And former Magpies caretaker and coach John Carver has been named as his assistant at the Hawthorns.
Pardew takes over from axed Gary Megson whose final act in the dugout was to oversee last night's 2-2 home draw with United.
In a statement on the Baggies official website Pardew confirmed his delight at landing the gig at "one of the great, traditional clubs of English football".
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He said: "I'm thrilled with the opportunity Albion have provided and looking forward to getting to work with what I consider to be a talented group of players.
"The immediate challenge will be to get the results we need to pull ourselves up the table. But I’m aware that while I’m joining one of the great, traditional clubs of English football, it is one determined to go forward in the Premier League.
"It’s an exciting opportunity to play a part in that ambition."
Pardew spent four years at United between 2010 and 2014, where he was replaced by Carver.
Despite one fifth placed finish in 2012, and an LMA manager of the year award, Pardew's Tyneside tenure is not fondly remembered by United fans.
He left for Crystal Palace where he spent 12 months before being sacked last year.
Albion chairman John Williams, added: "Alan brings the experience of more than 300 Premier League games and the kind of dynamic leadership from which our Club can benefit.
"We were impressed with what he had to say and what he has to offer and we are looking forward to an exciting new era under his charge."