Koplik's spots (also Koplik's sign) are a prodromic viral enanthem of measles manifesting two to three days before the measles rash itself. They are characterized as clustered, white lesions on the buccal mucosa (opposite the lower 1st & 2nd molars) and are pathognomonic for measles. The textbook description of Koplik spots is ulcerated mucosal lesions marked by necrosis, neutrophilic exudate, and neovascularization. They are described as appearing like "grains of salt on a reddish background", and often fade as the maculopapular rash develops. As well as their diagnostic significance they are important in the control of outbreaks. Their appearance, in context of a diagnosed case, before they reach maximum infectivity, permits isolation of the contacts and greatly aids control of this highly infectious disease.

Nobel laureate John F. Enders and Thomas Peebles, who first isolated the measles virus, were careful to collect their samples from patients showing Koplik's spots.