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

Pseudolycaena marsyas has a wingspan of about 40–50 millimetres (1.6–2.0 in), a quite huge size in hairstreaks (hence the common name "giant hairstreak"). The uppersides of the wings are usually metallic blue, with hues varying from cobalt blue to turquoise blue depending on location. 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.

• Austin, G.T. ; L.D. Miller & J.Y. Miller, 2007: Taxonomic comments on Pseudolycaena Wallengren (Lepidoptera: Lycaenidae: Theclinae: Eumaeini). Bulletin of the Allyn Museum 149: 1-22.
• "Pseudolycaena Wallengren, 1858" at Markku Savela's Lepidoptera and Some Other Life Forms

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