Martha-Renee Kolleh said she decided to do something because so many people would come in to the Yeanon Cafe in Ossett, West Yorkshire, and then leave immediately when they saw her skin color, the BBC reported.
The sign is straightforward: "I am a black woman.... If you are allergic to black people, don't come in."
Kolleh said there are plenty of people in Ossett who have no problem with her race. But she said there are enough of the other kind to hurt her business, which she opened two years ago, and told the BBC she will have to close in the fall unless things pick up.
"I hired a white woman called Tracey. When Tracey is in the cafe she does well and people will come in and it will be full, once I stay in the background," she said. "But if Tracey is not working and I'm working nobody comes in."
The Ossett area is overwhelmingly white, the BBC said.