Plants erect herbs, up to 1.5 m tall. Stems up to 8 mm thick, striate, brown, glabrous; achreae membranous, up to 25 mm long, truncate, glabrous, with or without a short fring. Leaves petiolate; petioles very short; blades up to 20 cm long and 3 cm wide, mostly glabrous on both surfaces but lower surfaces with numerous small glands midvein sometimes with sessile glands; bases attenuate; margins sometimes weakly strigose; tips attenuate. Inflorescences panicles of 2-several spikelike racemes, 2-8 cm long; peduncles usually glabrous, occasionally with a few sessile glands; bracts broadly ovate, truncate, not fringed, glabrous or with a few sessile glands. Tepals 5, 2-3 mm long, pink; styles 3, united for half their length. Nuts 2(-2.5) mm long, bluntly trigonous to lens-shaped, black-brown, glossy.
Polygonum glabra grows in wet and damp places such as along streams, on river banks, and even under water and may become established around artificial sources of water. It grows at elevations of 750-2900 m. It was known, at the time the Flora of Ethiopia and Eritreat was written from several districts in Ethiopia, Socotra, Malaysia, mainland Asia, and central and tropical America. In 2021, it was tentatively identified from western Somaliland.