Although data from the internal oral anatomy and chondrocranium of anuran larvae can be used to resolve taxonomic questions, there is no information on these characteristics for species of the Dendropsophus marmoratus group. Herein, we provide a complete description of the tadpole of D. soaresi, presenting additional information about the external larval morphology and unpublished data on internal oral anatomy and chondrocranium of specimens collected in the state of Piauí, northeastern Brazil. The measurements and morphological characters were based on 10 tadpoles in stages 35–36. Two tadpoles in stage 36 and one in stage 35 were dissected for analysis of the internal oral anatomy. To describe the chondrocranium, six stage 36 tadpoles were cleared and double-stained. We find small differences concerning the original description of the external morphology, such as body shape, oral disk size, shape and height of the fins, and position of the vent tube. We also present additional information on coloration in life and morphometric details. Similar to other species of the genus, the tadpole of D. soaresi has a reduced number of structures in the buccopharyngeal region, although presenting lingual papillae and a rounded cornified structure in the prenarial arena. Also, D. soaresi presents a set of chondrocranial traits that are not found in other species of the genus and are possible synapomorphies for the D. marmoratofus group. This variation highlights the potential of larval morphology as a source of evidence for Dendropsophus systematics. However, additional research on the internal anatomy of Dendropsophus species, especially those of the D. marmoratus group, are essential to clarify the phylogenetic relationships of the genus.