Alcalcus rajae is a medium sized, stocky frog. It reaches 40 mm (female) in snout-vent length.

It is known from Kalimantan. Coloration goes from light to dark brown. The adhesive disks bear a white cross-bar. 

The species differs from Alcalcus baluensis most notably in being substantially larger and lacking external visibility of the tympanum. Otherwise both species are similar in body shape, adhesive disks, and granular skin texture.

The species was discovered in a seepage area close to a slow moving stream in a swampy forest section.

Fotos of adults from West Kalimantan by ©Mediyansyah.