The 20-year-old forward posted a screenshot of the offensive message on his Instagram story, with the abuser calling him the N-word that was sent through a private message.
But the former Liverpool player appeared unfazed, as he wrote: "I bet u this kid is about 12" followed by laughing emojis.
The disgusting message read: “Overrated n***** you’re s*** poo boy.”
This is not the first time the young player has received racist abuse.
During a UEFA Youth League clash between Liverpool and Spartak Moscow's under-19s in 2017, Brewster opened up about being a victim of a racist attack.
He said one of the opposing players called him the derogatory N-word and a "negro" during the game.
Brewster immediately lodged a complaint with the fourth official after the referee had initially suggested delaying further action until the game ended.
His team mate David McGoldrick suffered similar abuse last summer, where a message was sent to his public Instagram page by an account without a name or a face.
And the club immediately vowed to support the player, and “do all they can” to identify the individual who sent a vile racist message to the Blades striker on Instagram.
Former United manager Chris Wilder also spoke out against the abuse aimed at Blades players this season.
He condemned the social media smears as "cowardly, disgusting and inhumane", adding that the individuals responsible "need nailing".
He added that the matter was already being investigated by the police.