She won the trophy thanks to her role of the tough, damaged lawyer and law school professor Annalise Keating on the ABC show, reported Us magazine.
The 50-year-old actress looked shocked and overwhelmed when her name was announced.
Wearing a Carmen Marc Valvo gown, the two-time Oscar nominee took the opportunity to make a statement about racial inequality in Hollywood and to thank the people, who helped get her into a leading role.
"So here's to all the writers, the awesome people -- people who have redefined what it means to be beautiful, to be sexy, to be a leading woman, to be black."
She got a standing ovation from actress Taraji P Henson, who was nominated in the same category for her role on TV show "Empire".