The 25-year-old made 36 appearances in Serie A for the Nerazzurri last season, netting five league goals as they finished fourth to qualify for the Champions League, beating out rivals Milan by a single point.
Having fully established himself as a core member under Luciano Spalletti, the club – now coached by former Chelsea man Antonio Conte – have reportedly paid €20m (£17.8/$22.6m) to seal his services full-time.
Coming through the ranks at Roma as a youth, Politano made his senior debut for Perugia in 2012 on loan from the Giallorossi before moving to Pescara.
Two seasons of eye-catching performances in Serie B brought him to Sassulo for three years, where he was rewarded with a senior call-up in 2018, making his debut in May last year in a friendly against Saudi Arabia.
Inter subsequently brought him to San Siro on a one-year loan deal with an option to buy, which the club activated following his string of key performances.
He scored his first goal for the club in a 2-0 win over Cagliari last September and went on to net against Genoa, Fiorentina, SPAL and Chievo, as well as netting his first international goal against the USMNT in November.
Inter also announced on Wednesday the signing of Eddie Salcedo from Genoa on a permanent basis after he too spent last season on loan with the club.
The 17-year-old is yet to make a senior breakthrough but has become a key member of Italy’s under-19 team since last year.
Goalkeeper Andrei Radu meanwhile is expected to go the other way after a temporary spell at Stadio Luigi Ferraris.