The Chinese and Iranian foreign ministers on Saturday signed a 25-year cooperation agreement between the two allies in a ceremony carried live on state television.
China has agreed to invest $400 billion in Iran's economy over that time period in exchange for a steady and discounted supply of oil from Tehran.
Investments will be made in dozens of fields in including banking, telecommunications, ports, railways, health care and IT.
"Our relations with Iran will not be affected by the current situation, but will be permanent and strategic," Chinese Foreign Minister Wang Yi was earlier quoted by Iranian news agencies as saying.
"Iran decides independently on its relations with other countries and is not like some countries that change their position with one phone call."
Wang met President Hassan Rouhani ahead of the signing of the agreement, which is expected to include Chinese investments in key sectors such as energy and infrastructure.
The accord is an example of “successful diplomacy,” Rouhani’s adviser Hesameddin Ashena was cited by Iranian media as saying. "A country’s strength is in its ability to join coalitions, not to remain isolated.”
Saeed Khatibzadeh, an Iranian foreign ministry spokesman, said the document was a “roadmap” for trade, economic and transportation cooperation, with a “special focus on the private sectors of the two sides.”
China, one of Iran’s largest trading partners and a long-standing ally, agreed in 2016 to boost bilateral trade by more than 10 times to $600 billion over a decade.
Its commerce ministry said on Thursday that Beijing will try to safeguard the 2015 Iran nuclear deal and defend the legitimate interests of Sino-Iranian relations.