"While I very much appreciate PM [Japanese Prime Minister Shinzo] Abe going to Iran to meet with Ayatollah Ali Khamenei, I personally feel that it is too soon to even think about making a deal", Trump said in a Twitter message. "They are not ready, and neither are we!"
Abe is currently in Tehran for a three-day visit to help ease the mounting tensions between the Islamic republic and the United States.
Earlier in the day, Abe held a meeting with Iranian Supreme Leader Ayatollah Ali Khamenei.
In May, Iran announced that it had partially discontinued its commitments under the JCPOA agreement and gave Europe 60 days to ensure Iran's interests were protected under the deal.
After the United States quit the landmark nuclear deal, tensions between Tehran and Washington went into a downward spiral, with the United States unveiling batches of sanctions against the Islamic republic that targeted the country's finance, transport and military sectors, among others.