It was the second time Trump certified Iranian compliance with the agreement since he took office in January, despite criticizing it during the 2016 campaign as "the worst deal ever."
Trump administration officials, briefing reporters on Monday on the decision, said new economic sanctions against Iran were being prepared over its ballistic missile program and for contributing to regional tensions.
Under U.S. law, the State Department must notify Congress every 90 days of Iran's compliance with the so-called Joint Comprehensive Plan of Action (JCPOA). Trump had faced a congressionally mandated deadline of Monday to decide.
A senior administration official said Iran was judged in compliance of the 2015 nuclear deal but that Trump and Secretary of State Rex Tillerson believed Iran "remains one of the most dangerous threats to U.S. interests and to regional stability."
The official ticked off a list of accusations about Iranian behavior in the region, including ballistic missile development and proliferation, support for terrorism and militancy, complicity in atrocities committed in Syria and threats to Gulf waterways.
"The president and the secretary of state judge that these Iranian activities severely undermine the intent of the JCPOA, which was to contribute to regional and international peace and security. As a result, the president, the secretary of state and the entire administration judge that Iran is unquestionably in default of the spirit of the JCPOA," the official said.