"The most important job of our foreign minister is first to stand behind the JCPOA, and not to allow the US and other enemies to succeed," Rouhani told parliament, using the technical name for the 2015 deal that eased sanctions in exchange for curbs to Iran's nuclear programme.
"Standing up for the JCPOA means standing up to Iran's enemies," he said on the last day of debates over his cabinet selections.
Rouhani indicated a week ago that Iran was ready to walk out on the nuclear deal if the United States continued to apply fresh sanctions.
It has come under mounting pressure after Tehran carried out missile tests and Washington imposed new sanctions -- with each accusing the other of violating the spirit of the agreement.
But Rouhani has insisted the deal remains the preferred way forward, not least to help rebuild Iran's struggling economy and create jobs.
He was addressing lawmakers ahead of votes of confidence on his ministerial choices.
Rouhani, a political moderate, began his second term earlier this month after winning a resounding victory over a hardline challenger in May.
He has vowed to continue his outreach to the world, and to improve civil liberties at home, but has been criticised by reformists for again failing to select a single female minister.
(Except for the headline, this story has not been edited by NDTV staff and is published from a syndicated feed.)