Capellan acid is a type of strong acidic liquid substance that naturally occurs on the planet Capella IV, in the Capella star system.

Capellan acid is present in small amounts in the water supply accessible to the Capellan civilization, giving them a natural immunity to Andronesian encephalitis. In the 2260s, Leonard McCoy, who had studied the Capellans, theorized that the Capellan acid might have other beneficial uses. Along with Doctor Lewis Rosenhaus, McCoy worked with this acid to neutralize xelaxine toxicity in humanoid bodies, with the breakdown products from the introduction of xelaxine to Capellan acid being carbon dioxide and ascorbic acid. (TOS - The Brave and the Bold, Book One novella: The First Artifact)