Napoli’s move for Lukaku, 31, has been driven by their manager Antonio Conte, under whom the Belgian enjoyed two prolific seasons for Inter Milan from 2019 to 2021.
The striker rejoined Chelsea in 2021 for £97.5m but has spent the past two seasons on loan in Italy, first back at Inter and then with Roma.
Lukaku scored 24 league goals as he won Serie A under Conte in 2021, with his form sparking Chelsea’s decision to recruit him.
Disposing of the Belgian – who has not played for Chelsea since May 2022 – gets his £325,000-a-week wages off the books.
Speaking in a pre-match news conference, Conte said Lukaku could make his Napoli debut in Saturday’s game against Parma.
Lukaku’s departure comes as Chelsea midfielder Tino Anjorin also moves to Italy’s Serie A, joining Empoli on a three-year deal.
Chelsea and Saudi Pro League side Al-Ahli are both in talks to sign Nigeria striker Victor Osimhen, 25, which will help Napoli offset their spending.