Hydrophis cantoris, also known commonly as Cantor's narrow-headed sea snake and Cantor's small-headed sea snake, is a species of venomous sea snake in the family Elapidae.

The specific name, cantoris, is in honor of Danish zoologist Theodore Edward Cantor.

[3] H.cantoris has the following diagnostic characters: Head small, body long and slender anteriorly; scales on thickest part of body juxtaposed; 5-6 maxillary teeth behind fangs; 23-25 (rarely 21) dorsal scale rows at neck, 41–48 at thickest part of body (increase from neck to midbody 18–24); ventrals divided by a longitudinal fissure; prefrontal in contact with third upper labial; ventrals 404–468.

[4] Indian Ocean (Bangladesh, Pakistan, India, Myanmar (= Burma), Thailand, Malaysia), Andaman Islands.

[2] The preferred natural habitat of H. cantoris is the marine neritic zone, at depths of 20 m (66 ft) or less.