The family Leptotyphlopidae comprises about 120 species allocated in 12 genera including Siagonodon, the latter with four currently recognized species. Here we propose new diagnostic characters for the genus Siagonodon based on the analysis of variation of external morphology and comparisons with related taxa. In order to evaluate the diagnostic characters of the genus Siagonodon, representatives of other genera of the tribe Epictini occurring in South America were also analyzed (Epictia, Rena, and Trilepida). We examined 14 species of the family Leptotyphlopidae and analyzed 67 characters of external morphology. We propose six new diagnostic characters for the genus Siagonodon, including: rostral scale with enlarged base, terminal spine reduced or absent, and straight snout in lateral view. Furthermore, we provide new data on morphological variation and geographical distribution of the poorly known Siagonodon cupinensis, expanding the diagnosis of this taxon and improving comparisons with congeners.