Dendrobium speciosum is an epiphytic or lithophytic herb with spreading roots and cylindrical or tapered pseudobulbs long and wide. Each pseudobulb has up to seven, usually thick, leathery leaves originating from its top, the leaves long and wide. The leaves can remain on the plant for up to twelve years. The flowers vary in colour from white to bright yellows and there is considerable variation in the length of the flowering raceme, the number of flowers on it and the size of the flowers. The length of the flowering stem ranges from long and the number of flowers from two to two hundred. The flowers are long and wide. The dorsal sepal is longer than the lateral sepals but narrower and the petals are about the same length as the lateral sepals but only half as wide. The labellum has reddish purple spots or streaks and three lobes, the sides lobes erect and curved and the middle lobe pointed, rounded or more or less square. Flowering occurs between August and October for most varieties but some flower as early as May and others as late as November.
