A Sikh is a follower of the Earth religion known as Sikhi.
Lieutenant Marla McGivers identified Khan Noonien Singh as being from northern India, a region populated by Sikhs during the late 20th century. She described them as "the most fantastic warriors." (TOS: "Space Seed")
Khan would be a Sikh only in the ethnic sense since one requirement of males following the Sikh religion is to neither cut their hair nor shave their beards as an outward display of their faith. Khan, of course, has never been portrayed as anything other than clean-shaven. Further, other than in Marla McGiver's interpretive artwork, Khan has never been seen wearing a turban, which is another requirement of the Sikh faith. [1] And while it is possible to be Sahajdhari (a "slow adopter"), and exempt from the requirement of uncut hair and beard, [2]