Polynemus aquilonaris is a medium-sized species of threadfin which attains a maximum total length of 15.8 centimetres (6.2 in).

The lateral line is simple, extends from the upper part of the gill slit to the tail.

The white anal fin has a translucent posterior margin with a similar pattern on the tail.

[3] Polynemus aquilonaris was first formally described in 2003 by Hiroyuku Motomura from a specimen purchased at Samyan Fish Market which had been caught in the Chao Phraya.

[4] It had previously been considered that the paradise fish in north of Malaya were populations of the Polynemus dubius but Motomura showed that they were a separate species.