Fourteen names are assigned to the Sittasomus griseicapillus complex, yet no taxonomic revision has been conducted. We compared vocalization and morphology of the nominotypic population with those of southeastern South America (S. g. sylviellus and S. g. olivaceus), assessing potential divergences. Recordings were obtained from databases (WikiAves and Xeno-Canto) and analyzed based on loudsong duration, note number, pace, and minimum and maximum frequencies in Raven Pro 1.6.5. Morphology was compared through body color coding and morphometric analysis of specimens from Museu de Zoologia da Universidade de São Paulo. Using a caliper (0.01 mm), we measured culmen length, width, and height, and tarsus, remiges, and rectrices length. Multivariate analysis of variance (MANOVA) was used to test for differences in traits between populations, followed by Student’s t-test to assess distinctions within them. Some 38 recordings were examined (griseicapillus – 15; sylviellus – 23), revealing a statistically significant difference (F_1,2 = 3.4; p = 0.013) between populations. Song duration-related traits differed significantly (p = 0.017), with mean loudsong duration being shorter in griseicapillus (4.2 ± 0.9 s) than in sylviellus (5.5 ± 2.1 s), while note number was higher in the former (16.4 ± 5.1) compared to the latter (11.9 ± 5.9). The mean pace of griseicapillus was faster (3.9 ± 1.1) than that of the Atlantic population (2.5 ± 1.7). Some 40 specimens were analyzed (griseicapillus – 10; sylviellus – 28; olivaceus – 2). No morphometric differences were found (F_1,3 = 0.98; p > 0.05) among the three populations, but body coloration greatly differed. The upperpart and underpart of griseicapillus is grayish (2.5Y 4/2) whereas in sylviellus and obsoletus, these regions are olive-green (5Y 5/6). All populations are reddish brown (2.5 YR4/6 – griseicapillus; 2.5YR 3/4 – sylviellus) in remiges, rectrices, and uropygium. The significant vocal and morphological differentiation detected suggests that griseicapillus and sylviellus should be full species