"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.
But Rouhani has insisted the deal remains the preferred way forward, not least to help rebuild Iran's 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.
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.