The deal included an option for Chelsea to extend the loan by another season for 18 million euros ($20.5 million), or buy Higuain outright for 36 million euros ($41 million).
The 31-year-old Higuain scored a record 36 goals in 35 Serie A games in his one season under Sarri at Napoli, in the 2015-16 campaign. Since then, he has played for Juventus and been on loan this season at AC Milan, where he has scored eight goals.
"Gonzalo was our No. 1 target in this transfer window and arrives with a proven record at the highest level," Chelsea director Marina Granovskaia said of a forward whose first club in European soccer was Real Madrid. "He has previously worked with Maurizio to great effect and is familiar with how the coach likes to play."
Chelsea has struggled for goals in recent weeks, scoring just eight times in its last nine games. Strikers Alvaro Morata and Olivier Giroud are out of favor under Sarri, who has experimented by deploying Eden Hazard — usually a winger — as a center forward but to no great effect.
Hazard can now return to playing in his favored position on the left wing. Morata might be sold or loaned out.
Whether Higuain can fix Chelsea's attacking problems is open to question, with the striker having underperformed at Milan this season. He has looked a far cry from the player whose scoring exploits at Napoli prompted Juventus to spend a club-record 90 million euros (then $99 million) to sign him.
Sarri clearly thinks Higuain can be the answer. He said after his first match in charge at Chelsea that he regretted not being able to sign the striker in the offseason.
Higuain hasn't signed in time to face Tottenham in the semifinals of the English League Cup on Thursday. Chelsea trails 1-0 from the first leg.
His debut might come against Sheffield Wednesday in the FA Cup on Sunday.
"It's a team I've always liked that has a lot of history, a wonderful stadium and they play in the Premier League, a league I've always wanted to play in," Higuain said of Chelsea.
"I now hope I can give back that trust Chelsea have shown me out on the pitch."