Guyanancistrus longispinis is a species of catfish in the family Loricariidae that is of disputed classification.

It is native to South America, where it occurs in the Oyapock basin in French Guiana.

It is known to occur alongside the species Guyanancistrus brevispinis, Harttia fowleri, and Crenicichla ternetzi, as well as members of the genus Ancistrus.

The species reaches 10.4 cm (4.1 inches) in standard length and may be a facultative air-breather.

Despite this reclassification, a 2018 taxonomic review of Guyanancistrus recognized G. longispinis as a valid member of the genus.