The 22-year-old joined Barcelona from Fluminense on an initial loan deal in 2016, with that move being made permanent after he impressed for the club's B team.
However, Marlon spent last season on loan to Nice and saw that two-year deal cut short this summer, returning to Camp Nou one year earlier than planned.
Barcelona have now shipped the Brazil Under-20 international out to Sassuolo for €6m (£5.4m), although the deal includes a buy-back clause for the Spanish champions.
La Blaugrana will also receive 50% of any future sale of Marlon unless he plays 50 games for Sassuolo, at which point that clause will expire but the Italian outfit must pay Barca a further €6m (£5.4m).
Marlon made three senior appearances for Barcelona during his short time at the club, all of which came during the 2016-17 campaign.