The first-instar larva of Suphisellus rufipes (Sharp, 1882) (Coleoptera: Noteridae) is described and illustrated for the first time, including morphometric and chaetotaxic analyses of the cephalic capsule, head appendages, legs, last abdominal segment, and urogomphus. This is the first detailed larval description for the genus Suphisellus Crotch, 1873. First-instar larvae of S. rufipes can be separated from those of Suphis Aubé, 1836 and Hydrocanthus Say, 1823 by the combination of an elongate body, posterior tentorial pits contiguous to occipital foramen, antennomere 4 approximately half the length of antennomere 3, inner margin of mandible serrate, siphon elongate and pointed, and urogomphi fused along inner margin. Regarding chaetotaxy, Suphisellus differs from Suphis and Hydrocanthus in the absence of setae AB5, AB6, AB7, AB8, AB14 and AB16, likely due to the strong modification of the siphon for underwater respiration, and in the shape of setae MX4, MX5 and MX6 on the maxillary stipes, and of setae LA3 and LA6 on the prementum.