Capsula oblonga, the oblong sedge borer, is a moth of the family Noctuidae. The species was first described by Augustus Radcliffe Grote in 1882. It is found across southern Canada from the Maritimes to British Columbia, south to the Gulf of Mexico and southern California. The wingspan is 35–50 mm. Adults are on wing from June to September depending on the location. There is one generation per year. The larvae initially leaf mine and later bore the stems of Typha and Scirpus species below the water line.