Polyommatus icarus has a wingspan of 28–36 millimetres. This species is very variable, depending on location. Common blue males usually have uppersides in an iridescent lilac blue with a thin black border. Females are brown above with a row of red spots along the edges and usually some blue at the base of the wings; the extent of blue and brown is extremely variable; the upperside may be mostly blue, , but it always has red spots. Undersides have a greyish ground colour in the males and a more brownish in the females.
Both sexes have a row of red spots along the edge of the hindwings and extending onto the forewings, though they are generally fainter there, particularly in the males, where they are sometimes missing altogether.