The winger, who has not started a Premier League game for United since leaving for Old Trafford last summer, rejoins Palace 24 hours after Neil Warnock's return as manager was announced.
He told Sky Sports News HQ: "There's a nice vibe around and I'm happy to be here. I was happy when I was here before."
Warnock said: "I think Wilfried needs us like we need Wilfried. He’s been at a big club like Man United, and it is a big club, and he’s probably got a bit lost in the chambers there.
“He could have gone to three or four clubs and I think people think wrong of him at times with what they say about his attitude.
“Wilfried has always had a great attitude for me. He wants to learn he wants to do extra training so we are going to let him do that."
Palace sold Zaha to United for £15million in January 2013 but took him back on loan for the rest of their successful promotion campaign.
The 22-year-old won the second of his two England caps later that year but was sent on loan to Cardiff for the second half of the season after failing to establish himself under David Moyes.