The appointment was confirmed by the board of directors after shareholders approved the proposal.
Descalzi will take over from Paulo Scaroni, who held the position for nine years.
The Italian government, which owns 30% of Eni, announced their nomination of Descalzi last month.
The 59-year-old has served as COO of exploration & production since 2008.
Descalzi has worked for Eni since 1981 when he started as an engineer.