The yellowline goby grows to 5 cm (2.0 in) in length. It has a rounded snout and a long, slim body. The upper parts are black and the underside is grey gradually paling to white. Some fish have a bright yellow stripe running along each side from the eye to the tail, with often a yellow spot or short line on the snout. In juvenile fish, this stripe is shorter and extends just beyond the base of the pectoral fin.
The yellowline goby is always found living in association with a tubular sponge such as Verongia aerophoba or a massive sponge such as Neofibularia nolitangere and feeding on the parasitic worms Haplosyllis spongicola that live in large numbers on these sponges. It also eats other invertebrates that live on the seabed and plankton. Seen in Tori's Reef dive site in Bonaire, among other places, always inside sponges.