What species is Spock's father Sarek?

Spock's father, Sarek, is a Vulcan.

Sarek is a Vulcan astrophysicist and ambassador to the United Federation of Planets. He is married to Amanda Grayson, a human, and their son Spock is therefore half-Vulcan and half-human. Sarek's identity helps explain the cultural tension at the center of Spock's character: Spock inherits Vulcan logic and discipline while also experiencing human emotion.

Sarek first appears in the original-series episode “Journey to Babel,” portrayed by Mark Lenard. In that story, he and Spock have been estranged because Sarek wanted his son to attend the Vulcan Science Academy, while Spock chose Starfleet service instead. A medical crisis aboard the Enterprise forces father and son to confront their unresolved relationship.

Sarek is sometimes confused with a Romulan because Vulcans and Romulans share ancient ancestry and look similar on screen. They are nevertheless distinct peoples in Star Trek, with different political histories and cultures. Sarek's full name is given as S'chn T'gai Sarek, though he is almost always called simply Sarek.