"There is no need to renegotiate parts of the agreement because the agreement is concerning a nuclear program and as such is delivering," Mogherini told reporters following a UN meeting of the six powers that negotiated the deal with Iran.
"We have all agreed that all sides are implementing so far the agreement," she said.
US Secretary of State Rex Tillerson joined Iranian Foreign Minister Mohammad Javad Zarif for a first meeting with partners backing the 2015 deal that provides for sanctions relief in exchange for curbs on Tehran's nuclear program.
Trump is due to report to the US Congress by October 15 on whether he can certify that Iran is upholding its side of the accord, under which it accepted limits on its nuclear program.
In his address to the UN General Assembly, Trump on Tuesday called the nuclear deal "an embarrassment" for the United States and Tillerson later confirmed that the agreement must be "revisited."
But Mogherini, who chaired the meeting, argued that it would be unwise to re-open the deal at a time when the world is facing a nuclear threat from North Korea.
"We already have one potential nuclear crisis. We definitely do not need to go into another one," she said.
Other than Iran and the United States, the other signatories of the accord are Britain, France, Russia, China and Germany.