The lesser swamp warbler is a plain-coloured smallish bird 14–16 cm long and weighing around 20 g. Its upperparts are rich brown, and it has a white supercilium.

The long, strong bill has a slightly down-curved upper mandible; it is blackish-yellow with a yellower base.

It nests mainly from August to December, with the earliest nesters being those in the winter rainfall areas of the Western Cape Province.

The lesser swamp warbler is usually seen alone or in pairs, moving through wetland reedbeds, and clambering up and down reed stems.

The population size is believed to be large, and the species is not believed to approach the thresholds for the population decline criterion of the IUCN Red List (i.e. declining more than 30% in ten years or three generations).