It was up on the left field wall for about one batter in the fourth inning until umpires asked it be removed because it was in fair territory.
There was a spattering of boos from the crowd as Red Sox security forced the fans to remove it and escorted them from their seats. The game was not delayed.
Red Sox manager John Farrell saw the banner draped over the Green Monster.
The group branded themselves as "a group of white anti-racist protestors" in a statement to the Washington Post. "We urge anyone who is interested in learning more or taking action to contact their local racial justice organization", the statement read.
The person said the five were not associated with any group in particular although they all work in different groups in Boston fighting racism and white supremacy.
"There were originally about eight people involved who had this idea, and those eight people come from various organizing groups in the Boston area".
There's been a lot of talk about race this season in and around Fenway Park.
The Black Lives Matter movement was one of the group's inspirations.
He added that the group had been responding to the long history of white supremacy and racism across the US that is continuing to pervade all aspects of today's American culture.
During a visit to Fenway Park in May, Baltimore Orioles outfielder Adam Jones reported that he was subject to racists taunts from fans during a game against the Red Sox. "So we did this banner as a gesture towards that, to have a conversation about that".
Racism is still ingrained in the American society, it seems. But the stunt left many people baffled as they struggled to decipher what the message actually meant.