The US president said he would impose the “highest level” of economic sanctions on Iran.
Speaking in the White House, Mr Trump said despite his previous warnings, the “disastrous” deal must be reformed and the necessary changes had not been secured.
He said: “The United States no longer makes empty threats. When I make promises, I keep them.”
Previousl,y Mr Trump said that if he allowed the deal to stand, there would soon be a nuclear arms race.
He also says a constructive deal could easily have been struck at the time, but it wasn’t.
Trump is calling Iran a “regime of great terror.”
And he says that “no action taken by the regime has been more dangerous than its pursuit of nuclear weapons and the means of delivering them.”