De La Fuente, 19, is likely to join the French side on loan initially with a clause included for them to make the deal permanent next summer for around €5 million.
The two clubs are still negotiating the finer details of the transfer, including whether that clause will be optional or obligatory and if Barca will maintain an option to re-sign winger in the future.
ESPN reported in April that Barca were willing to listen to offers for De La Fuente this summer, with interest coming from Spain, Germany and France.
Following the arrival of Sergio Aguero and the possible signing of Memphis Depay, who Barca hope to secure on a free transfer from Lyon, De La Fuente is set to slip further down the pecking order under coach Ronald Koeman.
Sources told ESPN that both the club and the player agree that he's outgrown the B team, who play in the Spanish third division, and needs to play regular football next season.
Marseille, who are owned by the American businessman Frank McCourt, emerged as the frontrunner to sign him over the weekend.
Coach Jorge Sampaoli even phoned De La Fuente, who has been at Barca since 2013, to explain how he will fit into his plans at the French club and what his role would be.
Speaking to ESPN in May, De La Fuente -- who made his USMNT debut in November -- explained how difficult it had been at times this year bouncing between the first team and the B team, hinting that he would be open to a new challenge.