Located adjacent to Fenway Park, the mural faces the Massachusetts Turnpike and debuted ahead of the team's exhibition game against the Toronto Blue Jays.
The billboard was unveiled after the 61st anniversary of Elijah "Pumpsie" Green debuting as the Red Sox's first black player in 1959 — making Boston the last MLB team to integrate.
In June, the Red Sox displayed "Black Lives Matter" on the scoreboard at the baseball field to show their support.
"Our anger and grief must translate to action. We have a responsibility to use our platform to amplify messages of truth and justice," the team tweeted at the time.
Last month, Red Sox outfielder Jackie Bradley Jr was among several black MLB players to participate in a video supporting the Black Lives Matter movement.