In a statement after a board meeting, TIM said its board will meet on Oct.21 to discuss the appointment of a successor.
Board member Michele Valensise will take on the chairman role on an interim basis, the statement added.
French conglomerate Vivendi is TIM's main shareholder with a 24 percent stake, while U.S. activist fund Elliott and state lender CDP both have a stake of just under 10 percent.
Conti's resignation was expected after he said earlier this month he was considering stepping down in light of a "renewed climate of trust" between the company's shareholders.
(Reporting by Elvira Pollina, editing by Silvia Aloisi)
