Pseudolycaena marsyas, the Cambridge blue, giant hairstreak or Marsyas hairstreak, is a species of butterfly in the family Lycaenidae.

The apex of the wings are black and lightly falcade in males and the hindwings are tailed in both sexes, with a small black spot.

The undersides of the wings are pale blue greyish, with several black spots and thin markings.

This wide-ranging species occurs in Central and South America from Mexico up to Argentina, in semi-open forests at an elevation of about 0–1,000 metres (0–3,281 ft) above sea level.

Many forms varying in the hue of the blue but these are allocated to a single subspecies - Pseudolycaena marsyas marsyas.