Mr Guterres said the Iran agreement was an "important diplomatic victory" and should be maintained.
The UN secretary general said the Middle East would become an even more dangerous place if such an important diplomatic victory was thrown away with nothing better to replace it.
The US president is due to decide next week whether to withdraw from the agreement, by which Iran accepted to limit certain aspects of its nuclear programme in return for lifting of all nuclear-related economic sanctions.
Guterres added: "If one day there is a better agreement to replace it it’s fine, but we should not scrap it unless we have a good alternative."
The U.N. secretary general, also said there was a real risk of war if the 2015 agreement was not preserved.
