Procamallanus (Spirocamallanus) belenensis n. sp. is described using light and scanning electron microcopy, based on specimens taken from Ageneiosus ucayalensis, Castelnau, 1855 (Pisces: Siluriforme), a freshwater fish commonly called mandubé, in Pard State, Brazil; Procamallanus (Spirocamallanus) rarus is recorded from the same host. To our knowledge there are no previous reports of nematodes from this fish species. Procamallanus (S.) belenensis is characterized by the presence of 8-13 cuticular spiral thickenings coating 2/3 of posterior region of the buccal capsule of males and females (the anterior third is smooth); small sub-equal spicules; 3 and 4-5 pairs of pre- and post-cloacal papillae, respectively; the proportion of muscular and glandular esophagus is 1:1.5-2; males present 2 spike-like projections at the tail tip, which are absent in females by light microscopy. The most closely related species are Procamallanus (Spirocamallanus) inopinatus and Procamallanus (Spirocamallanus) pintoi. The new species differs from the most closely related species by the presence of 8-10 cuticular spiral thickenings in the buccal capsule of males and 9-13 in the females, 8 pairs of male clocal papillae (3 pre- and 5 post-cloacal, respectively), and only males bearing 2 minute, spike-like projections at the posterior end.