After meeting the head of Iran's Environmental Protection Organisation, Massoumeh Ebtekar, Royal said the two countries would build several key partnerships by February.
"Several highly operational subjects were discussed," Royal said.
She was travelling with senior business figures from environmental and renewable energy firms specialising in water, pollution and energy efficiency, including Isabelle Kocher, the boss of multinational Engie.
"They were chosen by Iran on the basis of the challenges they face," said Royal. "Since we face the same type of challenges in France, this is a terrific opportunity for cooperation."
She will return in February to monitor progress on the projects, she said.
Ebtekar, who is also a vice-president of Iran, said the visit "shows the important international cooperation on the environment and sends a strong message to the global community that countries can cooperate strongly on their shared interests and issues in the field of environment".
Domestic demand for power has weakened Iran's oil exports and forced the government to look into renewable energy.
Headscarf and burkini
At the press conference Royal had her hair covered, in compliance with Iranian law, and appeared reluctant to take many questions from journalists in the wake of international media coverage of bans on burkini swimsuits by 31 French resorts, according to the AFP news agency.
On Monday she was to visit Iran's largest saltwater lake, Orumiyeh in north-western Iran, a Unesco heritage site that has lost 90 percent of its surface water in the past two decades as a result of overfarming, dams and climate changes.
French companies have been pitching for business in Iran since the lifting of sanctions over the country's nuclear projects in January.
To read our coverage of last year's Cop21 climate change conference in Paris click here