The 30-year-old will return to Ligue 1 having begun his career at St Etienne. He joined Swansea on a free transfer in 2014 following six years at Olympique Lyonnais.
"Today is an important day in my career," Gomis said on Instagram.
"I am proud to wear to wear the colours of Olympique de Marseille. One of the great clubs in the French League."
Gomis enjoyed mixed fortunes at Swansea. He scored 17 goals in 71 games, but frequently flattered to deceive.
He arrived at the Liberty Stadium boasting an impressive scoring record at Lyon and brought physicality to the Swansea frontline.
The France international was expected to become a focal point in the Welsh club's attack when striker Wilfried Bony left to join Manchester City in January, 2015 but his form remained patchy.
He scored four goals in his first four league games last season, but managed just two more in the remainder of the campaign.
(Reporting by Toby Davis; editing by Ed Osmond)
