Rhacophorus kajau is a beautiful, small frog, reaching 20 mm in males and > 20 mm in females. It has a soft, high pitched call. It is easily overlooked in the field and was scientifically described only in 1984 by Julian Dring. However, this species is quite common in lowland habitats.
These frogs sit on vegetation, 1–2 m high. They prefer small streams with slow current for reproduction or even stagnant pools that become ephemeral streams during the wet season. Clutches with only a few eggs are attached to leaves overhanging water.
The tadpoles are easy to recognize. They have a dark brown saddle across the trunk. Body shape is ovoid. The iris is bronze with a red cross-bar. In close-up, there are conspicuous golden and silver pigments on the sides of the trunk. The skin of the belly is translucent.
The tail fins are well developed. The tail tip is moderately pointed (the specimen on the photo has a damaged tail tip, possibly the result of a dragonfly larva attack).