Pseudancistrus sidereus is described from two collections of specimens from Amazonas, Venezuela. Pseudancistrus sidereus differs from all other members of Pseudancistrus based on the presence of a unique keel on the caudal peduncle formed from the concave dorsal sections of the plates in the ventral series, light spots on the body (these may be yellow in life) centered on the body plates and the absence of the characteristics of the more derived members of Pseudancistrus such as hypertrophied odontodes along the snout in males and females and the loss of evertibility of the cheek plates. Pseudancistrus includes Lithoxancistrus and Guyanancistrus and is diagnosed by the following synapomorphies: no suture between pterotic-supracleithrum and hyomandibula, no contact of the hyomandibula with the prootic, straight, spoon-shaped anterior process of metapterygoid, nasal bone not much wider than laterosensory canal running through it, sphenotic not contacting posteriormost infraorbital externally, and a short ventral ridge on the pelvic basipterygium (lost in some species). Two species formerly assigned to Guyanancistrus are placed in different genera. Hypostomus guacharote is placed in Lasiancistrus and Chaetostomus trinitatis is placed in Ancistrus.