The northern mangrove seasnake (Parahydrophis mertoni), also known commonly as the Arafura smooth seasnake, is a species of venomous snake in the family Elapidae.

The species was first described in 1910 as Distira mertoni by Jean Roux.

The specific name, mertoni, is in honor of German zoologist Hugo Merton.

[1] Parahydrophis mertoni is blackish-olive with about 46 yellow rings on the body and ten on the tail.

The head shields are spotted with yellow, except for the rostral and labials which are black.