Herbs, epiphytic or rarely terrestrial, ± climbing, fan-shaped. Rhizomes often elongate, covered with bracts; pseudobulbs reduced to absent. Leaves: basal 3-7, apical not more than 1; blade recurved, lanceolate, laterally flattened-3-gonal, margins often serrulate. Inflorescences racemes or weakly paniculate; bracts narrowly triangular. Flowers: sepals and petals spatulate-clawed; lateral sepals sometimes partially connate; lip 3-lobed, pandurate, without nectar cavity; callus prominent tuberculate-lobulate; column with pair of outstretched apical wings, without hood; stigmatic cavity round; rostellum not prominent. Fruits capsules, ellipsoid.

Tolumnia does not belong to the main body of Oncidium (its traditional placement) even though many of the species have floral morphologies nearly identical to that of Oncidium. It shares, with other twig epiphytes, a fan-shaped habit and seeds with prominent extensions of the testa.