"Chelsea Football Club is delighted to announce Michy Batshuayi has today completed his transfer from Marseille," the club announced on their website.
Reports in France suggested the transfer fee was in the region of 40 million euro (£32.9m, $44.4m), although the Londoners are yet to confirm how much they paid.
Chelsea described the lightening quick striker as "powerful, fast and highly coveted and with a reputation as a fine finisher".
While he becomes the third Belgian international from the current Euro 2016 squad at the London club, he also follows in the footsteps of Chelsea legend Didier Drogba in switching Marseille for Stamford Bridge.
"Eden Hazard and Thibaut Courtois have told me many good things about the club," Batshuayi said of his compatriots.
"And with Antonio Conte coming in it's an exciting time to become a Chelsea player," he added.
"This is a fantastic opportunity for myself and my family, I am looking forward to playing in the Premier League next season," said Batshuayi, whose parents come from the Democratic Republic of Congo.
A 6.5-million-euro signing from Standard Liege in 2014, Batshuayi scored 17 goals in Ligue 1 last season, and 23 times in all competitions in a rather poor Marseille outfit that finished 13th in France.
The 22-year-old was however part of the Belgium squad at Euro 2016 and netted with his first touch on his tournament debut, converting a cross from his new Chelsea team-mate Hazard in a 4-0 win over Hungary.
Batshuayi spent his first season at Marseille as understudy to Andre-Pierre Gignac, before the France international left for Mexican side Tigres at the end of the last campaign.
Although he signed a contract extension in January, tying him to the club until 2020, he had been expected to leave this summer as Marseille scramble to balance their books or face a transfer ban.