Trump, a Republican, has never run for the party's nomination before, but has often talked about it.
He said his fortune would allow him to be an effective president, and if he gets elected president he will bring it back bigger and better and strong than ever before.
He said, he will make America great again.
He becomes the 12th Republican to declare, with Wisconsin Governor Scott Walker and Florida's former governor Jeb Bush among the early frontrunners.