Ahmadinejad, viewed by Western countries as a conservative, takes power next month after he won an election in June in the second-biggest crude exporter of the Organization of the Petroleum Exporting Countries (OPEC).
Some conservatives, who want domestic companies to work on Iranian projects, have been buoyed by comments from the new president that foreign companies will not have preferential treatment.
But Asefi, on a visit to London, sought to soothe the nerves of Western investors who may have preferred to see the former president Akbar Hashemi Rafsanjani win the election.
"Ahmadinejad has said he is open to foreign investment," Asefi told reporters. "He said he will not close the door, and he is thinking of creating more opportunities."
That meant strengthening relations with European countries as well as with China and Russia, he said.
Iran's parliament has already sent out negative signals to foreign firms.
Legislators have accused oil giant Royal Dutch Shell, which operates Iran's 200,000 barrels per day offshore Soroush and Nowruz oilfields, of "cultural imperialism".
Parliament also slashed the stake that Turkish mobile phone operator Turkcell was allowed to take in a mobile phone operating licence, potentially worth $3 billion.
"The worry in the West is based on a lack of knowledge. If foreign companies wait a bit, they will see there is nothing to be worried about"
Hamid Reza Asefi,
Iranian foreign ministry
Analysts see a $1.2 billion deal between Iran's National Petrochemical Company (NPC), Germany's Linde and Hyundai of South Korea as a test of whether Iran is ready for foreign investment.
Oil Minister Bijan Zanganeh has argued that Iran needs to award deals to more expensive foreign firms because only they have the technological know-how to do the job well.
Lack of knowledge
Zanganeh said on Tuesday that heavy foreign investment is needed to boost Iran's crude capacity, as the largest fields lose about 7% of capacity each year.
Asefi said Western firms were anxious because they did not know what to expect.
"The worry in the West is based on a lack of knowledge. If foreign companies wait a bit, they will see there is nothing to be worried about," he said.
He said Ahmadinejad would make economic and social policy a priority and that he aimed to help poorer Iranians.
Asefi met British Foreign Minister Jack Straw on his visit to London and the two agreed to greater cooperation on visas and on combating illegal immigration.