The news agency "Reuters" the Iranian minister, as saying, at an energy conference in Moscow: "Prince Abdul Aziz bin Salman friend for more than 22 years."
In September, Saudi Arabia's King Salman issued royal orders to appoint Prince Abdul Aziz bin Salman as his successor.
Prince Abdul Aziz has long been a member of the Saudi delegation to the Organization of Petroleum Exporting Countries (OPEC) and has experience in the oil sector for decades.
The appointment marks the first time a member of the Saudi royal family has served as energy minister in the world's biggest oil exporter.
Prince Abdul Aziz was appointed Minister of State for Energy in 2017, and worked closely with former Oil Minister Ali al-Naimi as his deputy for years.
The oil portfolio has held five ministers since 1960, none of whom were members of the ruling family. Last month, Saudi Arabia set up a ministry of industry and mineral resources, separating it from the huge energy ministry.