The 34-year-old full-back has been without a club since being released by Premier League leaders City at the end of the 2016-17 campaign.
Various landing spots have been mooted for the experienced France international since dropping into the free agent pool, but he has taken the decision to head for Italy.
Sagna has penned an initial short-term contract through to the end of the season.
The deal does, however, include the option for a 12-month extension through to the summer of 2019 if all parties wish to prolong their association.
“In the presence of president Oreste Vigorito, the French defender of Senegalese origins signed a contract that binds him to the Giallorossi club until June 30.
“The deal includes the option for the extension until June 30, 2019. Bacary Sagna will be available to coach Roberto De Zerbi from next week.”
There is little chance of him challenging for more honours with Benevento any time soon, as they have endured a humbling step up to the Italian top-flight.
Rooted to the foot of the table, the Serie A minnows have collected just seven points from 22 fixtures to date.