"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.
US President Donald Trump repeatedly threatened to tear up the deal during his campaign, and 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 it remains Iran's preferred way forward, not least to help rebuild the struggling economy and create jobs.
"The second responsibility of the foreign ministry... is to get involved in economic activities. It should help attract foreign investment and technology," Rouhani said, adding that Iran needed $200 billion in investments for the oil and gas sector alone.
Parliament approved 16 of his 17 cabinet picks, rejecting his suggested minister of energy, a reformist named Habibollah Bitaraf.
Foreign Minister Mohammad Javad Zarif, who was the charismatic face of Iran's nuclear negotiations, retained his position.
So did Oil Minister Bijan Namdar Zanganeh, who recently struck a billion-dollar deal with French giant Total.
Rouhani, a political moderate, worked hard behind the scenes to secure support for his choices, including from the supreme leader and the military.
He began his second term earlier this month after winning a resounding victory over a hardline challenger in May, vowing to continue his outreach to the world and improve civil liberties at home.
But he has angered reformists by again failing to appoint a single woman minister, and looks no closer to securing the release of jailed opposition leaders -- one of whom, Mehdi Karroubi, briefly went on hunger strike this week to demand a trial after six years under house arrest.
Rouhani has yet to appoint a minister of science, research and technology, which conservatives consider to be a sensitive post.