"Napoli is pleased to announce the signing of Benoit Badiashile from Chelsea... on an initial loan move with an option to purchase," the Serie A side said in a statement.
The 25-year-old centre-back joined Chelsea from Monaco in January 2023 in a deal reportedly worth 37 million euros (£31.5m).
Badiashile made 71 appearances in all competitions for the Premier League club, scoring twice, but an injury-hit campaign limited him to just eight league appearances last season.
He joins a Napoli side that won the Serie A title in 2025 and finished second last season behind Inter Milan.
The club changed coaches during the close season, with Antonio Conte departing and Massimiliano Allegri taking charge.
Napoli open their Serie A campaign at home to Genoa on Saturday.