The Guianan Tyrannulet (Zimmerius acer) is a species of bird in the Tyrannidae family, the tyrant flycatchers. It is found in forests in the east Amazon Basin and north-eastern Brazil. Until recently, it was considered a subspecies of the Slender-footed Tyrannulet. Following the split, it has not been rated by BirdLife International, but as it is widespread and genererally fairly common, it is unlikely to be threatened.