The transition from childhood into adolescence is associated with marked increases in testosterone, a sex hormone that has been linked with significant changes in brain structure and function. However, the majority of the extant literature on sex hormone effects has focused on structural brain development, with far fewer studies examining changes in the neural dynamics serving higher-order cognitive function and behavioral improvements with development. Herein, we investigated whether the neural oscillatory dynamics serving selective attention were sensitive to testosterone levels as a marker of development in a sample of 87 participants aged 6-13 years old. Participants completed a number-based Simon task while undergoing magnetoencephalography (MEG) and the resulting data were transformed into the time-frequency domain, imaged using a beamformer, and analyzed using whole-brain analysis of covariance models. Our key findings included spectrally-specific alterations in alpha and gamma oscillatory power in the prefrontal, parietal, and temporal regions with developmental shifts in testosterone levels, after accounting for the effect of age. Additionally, sex-by-testosterone interactions were found in the anterior cingulate, prefrontal, and parietal cortices that may indicate sexually divergent brain network development during the employment of selective attention. In sum, these results provide crucial new evidence supporting a relationship between developmental changes in testosterone and functional brain dynamics in youth during a critical period for skill acquisition and refinement.