The 21-year-old striker is Ian Holloway's first signing as Eagles boss and goes straight into the squad for Saturday's trip to Burnley.
Nimely has appeared in one Premier League game for City, against Burnley in 2010, and has had spells on loan with Middlesbrough and Coventry.
He told the Palace website: "I've played against Blackpool and Crystal Palace a couple of times so I think the manager has been watching me for a long time, so when he gave me the call to come here I was very happy to.
"The lads are all good and they've welcomed me well. I trained last week before the game against Stoke and I watched that game and was shocked by the way they play football by keeping the ball on the floor.
"I met the manager and he's fantastic. His philosophy is great as he wants to play good football and help the young players improve."
Holloway added: "I've admired him for a couple of years, I like his movement, skill and his tenacity. I want him to play for us and get some goals and hopefully he'll do that.
"He's a nice kid and he's one of the ones that they tend to stack and rack at clubs like Manchester City and I'm very pleased that with the choices he had he's chosen us."