The 21-year-old signed for United from Palace in January 2013 in a £15m deal, then finished the season on loan at his old club, helping them win promotion.
He made only four appearances for the Red Devils and joined Cardiff on loan for the second half of last season.
"I'm just buzzing to be back, to be in and around the lads again," he said.
"There have been rumours for a while, but it was only yesterday that it got sorted out and I am happy because I just need to get out and play.
"The best place to come back to is Palace where I am loved."
Zaha came through the youth ranks at Palace and was helped in his development by Neil Warnock, who returned to the south London club for a second spell as manager on Wednesday.
Warnock was previously in charge between 2007 and 2010, although he had left for QPR by the time Zaha made his first-team debut against Cardiff in March 2010.
Zaha went on to score 18 goals in 143 appearances.
Crystal Palace have re-signed winger Wilfried Zaha from Manchester United on a season-long loan.