Two adult Horvathinia pelocoroides Montandon 1911 (Hemiptera, Belostomatidae, Horvathininae) are recorded for the first time in a true wetland habitat. This species, probably the only valid ones in the monobasic subfamily Horvathininae Lauck and Menke 1961, was discovered in a permanent shallow pond near the city of Corrientes, Corrientes Province (27° 28′S, 58° 50′W), Argentina, during surveys conducted on December 29, 2004, and September 23, 2005. At both dates, the specimens were taken from a fresh water marsh densely filled with floating hydrophytes dominated by water hyacinth, Eicchornia crassipes, and water lettuce, Pistia stratiotes. This is the first record of any horvathinine specimen collected in any situation other than flying to light.