Iran, which is the world's fourth-largest producer of crude oil, has been without top-level direction for its oil policy since August, with output capacity sliding and no new production deals signed.
Kazem Vaziri-Hamaneh, the nominee approved Sunday, was the acting oil minister and served as a deputy oil minister under former President Mohammad Khatami, whose reformist allies Mr. Ahmadinejad defeated in elections in June.
Parliament refused to approve Mr. Ahmadinejad's first three nominees. Members complained that the president was not consulting them and was just picking close allies who did not have a deep knowledge of the industry.
Lawmakers speaking in Mr. Vaziri-Hamaneh's favor praised both his experience and his lifestyle, which they said conformed to the ideals of the Islamic revolution of 1979.
Mr. Vaziri-Hamaneh's policies are not yet clear, but he has said he will change "buy back" deals, the standard contract for foreign investors wanting a portion of oil and gas reserves.
Under those contracts, foreign firms operate Iranian oil fields for a short period and are compensated for their investment with an output share. The state then repurchases the field, after a period that foreign firms complain is too short.
Mr. Vaziri-Hamaneh has said, though, that foreign firms will still be welcome. Some of those arrangements have gone sour for Iran.
Iran has been at odds with Royal Dutch Shell over the offshore Soroush and Nowruz oil fields, which Shell developed with JJI of Japan and the Oil Industries Engineering Company of Iran and has since handed back to Iran.
When technical problems forced the fields' temporary closure earlier this year, Iran blamed Shell. It said some work was of poor quality and subcontractors were not properly supervised.
Shell rejected the charge.
Mr. Ahmadinejad made oil a cornerstone of his campaign for president in June.
He said he would distribute oil wealth more fairly, favor domestic investors over foreign one and combat the "mafia" that he says runs Iran's oil industry.
Shell, Total, Statoil and ENI are involved in Iranian projects.
Mr. Vaziri-Hamaneh also said he would try to shift the emphasis of Iran's exports from crude to refined products.