Pseudancistrus sidereus is a species of armored catfish known only from the upper Orinoco basin in Amazonas state, Venezuela. This is a fairly large loricariid (up to 17.6 centimetres (6.9 in) standard length), and is distinguished from its congeners by a strong keel on the caudal peduncle and the colour scheme which is generally very dark with bright white or yellow spots on the upperside.