Nanobagrus immaculatus, a new species of bagrid catfish, is described from the Kahayan River drainage in southern Borneo. Nanobagrus immaculatus is most readily distinguished from congeners in having a uniformly dark-colored body lacking pale spots or patches (vs. pale spots or patches always present), larger eye (16.7–18.2% HL vs. 9.0–16.1), and a longer adipose-fin base (24.9–26.6% SL vs. 12.8–21.6). The synapomorphies used to diagnose Nanobagrus are reviewed and re-examined. Pseudomystus fuscus is hypothesized to be more closely related to Nanobagrus than to all other Pseudomystus and is hereby reassigned to the former genus.