Trees, shrubs or herbs, sometimes climbers or epiphytes. Stems, trunks and branches unarmed, occasionally thorny; ant-epiphytes with conspicuous hypocotyl tubers. Leaves opposite (decussate) or sometimes in whorls of 3(4) or more, margin entire; stipules mostly interpetiolar, rarely leafy. Inflorescences terminal and/or axillary, basically thyrsic to paniculate, often variously modified, very many- to 1-flowered. Flowers very small to very large, (3-4)5(12)-merous, hermaphrodite, seldom unisexual, actinomorphic, occasionally slightly zygomorphic, rather frequently heterostylous. Calyx inserted at apex of ovary, differentiated into a basal tubular portion and free, fairly large to minute or subobsolete lobes, or tubular portion reduced and consisting of discrete lobes only (some of which occasionally enlarged and foliaceous or petaloid). Corolla sympetalous, tubular to campanulate, seldom (sub)rotate, lobes mostly valvate or contorted, less often imbricate in bud. Stamens as many as and alternating with the corolla lobes, with very short to long filaments; anthers dorsi- or basifixed, introrse, made up of 2 thecae, normally dehiscing lengthwise, rarely with apical pores. Ovary inferior, very rarely (half-)superior, syncarpous, 2(-5, rarely more)-locular, mostly crowned by a variously shaped nectar-producing disk; placentas axile, basal or apical or (seldom) parietal (ovary then 1-celled); ovules mostly anatropous, 1 to many per locule. Style mostly simple, less often split into style branches; stigmas capitate, clavate or lobed. Fruits either dehiscent and dry (capsules) or indehiscent and fleshy (drupes, berry-like, rarely true berries), seldom indehiscent and dry or splitting into mericarps; fruits occasionally united to form an infructescence or “syncarp” (then ovaries of adjoining flowers of an inflorescence fused as well). Seeds mostly with copious entire or ruminate endosperm, occasionally variously winged; embryo straight or curved, often quite small.
Character states not applicable to Thai Rubiaceae hae been omitted.