Other forms: segregationists
Someone who believes that people of different races shouldn't live, work, or go to school together is a segregationist.
As the laws about racial segregation have changed, it's become less and less socially acceptable to openly be a segregationist. Before the 1950s, there were many white segregationists who actively opposed the integration of schools, workplaces, and neighborhoods, and until 1967 it was still illegal for black and white people to marry each other in some US states. Segregationist comes from the Latin root segregare, "set apart, isolate, or divide."