The 21-year-old joined United from Palace for 15 million pounds in January last year, but returns to Selhurst Park having failed to break into the first-team at Old Trafford.
"I'm just buzzing to be back, to be in and around the lads again," he said on the Crystal Palace website.
"There have been rumours for a while, but it was only yesterday that it got sorted out and I was happy it got sorted out ASAP because I just need to get out and play.
"The best place to come back to is Palace where I am loved."
Zaha, who has two England caps, made four appearances in all competitions for United and spent the latter half of last season on loan to Cardiff City, who were relegated from the top flight.
He was part of the Crystal Palace team that secured promotion to the Premier League in 2013, having stayed at the club on loan after signing for United, and was voted young player of the year in two consecutive seasons during his first spell at the London club.
(Reporting By Sam Holden; editing by Toby Davis)
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