"This is the worst deal. We got nothing," Trump told Fox News in reference to the 2015 accord negotiated with Iran by the United States and five other world powers.
It gave Iran sanctions relief in exchange for curbs on its nuclear program in a bid to prevent it from developing nuclear weapons.
"We did it out of weakness when actually, we have great strength," said Trump.
"We will see what happens pretty soon," said Trump, who must announce his decision on whether to certify Iran's compliance by the end of the week.
Every 90 days the president has to notify Congress as to whether he believes Iran is complying with the accord and if the lifting of sanctions is in the interest of the American people.
So far Trump has certified the accord but said the next deadline on Sunday is the crucial one.
Several US officials have said Trump might this time choose not certify the accord.
If he chooses not to certify, Trump would be defying the opinion of some of his top advisers, European countries and the International Atomic Energy Agency.
A decision by Trump to decertify the deal would leave it at grave risk, with the US Congress having 60 days to decide whether to re-impose specific sanctions on Tehran that were lifted because of the diplomatic pact.