The 22-year-old scored 23 goals for the French club last season and was part of the Belgium side which reached the last 16 at Euro 2016.
He joins fellow countrymen Thibaut Courtois and Eden Hazard at Stamford Bridge.
"I am delighted to sign for one of the biggest clubs in Europe and embark on the next chapter in my career," he told the club's website.
"I hope I can help Chelsea win many trophies during my time at Stamford Bridge.
"Eden Hazard and Thibaut Courtois have told me many good things about the club, and with Antonio Conte coming in it's an exciting time to become a Chelsea player.
"This is a fantastic opportunity for myself and my family, I am looking forward to playing in the Premier League next season."
Marseille is reportedly selling Batshuayi in order to reportedly avoid a transfer embargo.
Chelsea has signed a number of Belgian players in recent years, notably Hazard, Kevin De Bruyne and Romelu Lukaku, though the latter duo were both sold in 2014.
Batshuayi didn't have much time to impress with Belgium at Euro 2016, spending the majority of the time on the bench but did score in the 4-0 win over Hungary.
The forward will provide competition for Diego Costa, who had a slow second season with the club, scoring 12 league goals compared to his tally of 20 during his debut campaign.
Chelsea finished 10th in the Premier League last campaign in a season which was marred by the sacking of Jose Mourinho, with Guus Hiddink stepping in as interim manager.