Socio-communicative impairments are among the most salient features of autism spectrum disorder (ASD). It has been suggested that atypical connectivity plays an important role in ASD. One brain region that may be affected, the posterior superior temporal sulcus (pSTS), has been implicated in the processing of language, biological motion, and social context. Abnormalities in the development of pSTS may therefore account for some of the socio-communicative deficits in ASD. In the present study, we investigated functional and anatomical maturation of pSTS with functional connectivity MRI (fcMRI) and anatomical measures, obtained by automated cortical parcellation. FcMRI was used to examine intrinsic, low frequency BOLD fluctuations of pSTS subregions in 47 children and adolescents 8-19 years old during continuous performance on a visual search task. Twenty-one participants with ASD and 26 typically developing (TD) individuals were matched on age, sex, and IQ. Three functional subregions of pSTS were delineated in each hemisphere with a data-driven approach based on principle component analysis, and whole-brain connectivity maps from each of the six pSTS fcMRI seeds were created. A differentiation index for pSTS was computed in each individual as a coarse measure of functional maturation through the development of distinct network connections. We found that two subregions in rostral and caudal pSTS participated in overlapping networks, which were both topographically and temporally distinct from a third region located between the two in mid-pSTS. In direct-group comparisons, the networks subserved by pSTS were significantly less differentiated in ASD. This was reflected in regions of increased connectivity relative to the TD group. There was a significant positive association between the differentiation of networks with age in the TD group that correlated with cortical thinning in pSTS and whole-brain white matter volume. In the ASD group, on the other hand, differentiation of pSTS connectivity did not correlate reliably with measures of anatomical maturation, but was inversely related to symptom severity. Atypical maturation of pSTS suggests altered trajectories for functional segregation and integration of networks, which would have ramifications in the development of cognitive and sensorimotor processing. In addition, the present study provides a novel explanation for atypically increased connectivity in ASD observed in some fcMRI studies. Our findings highlight the importance of interpreting group differences in ASD fcMRI studies from a neurodevelopmental perspective.