Objective: Previous studies examining selective attention in individuals with autism spectrum disorder (ASD) have yielded conflicting results, some suggesting superior focused attention (e.g., on visual search tasks), others demonstrating greater distractibility. This pattern could be accounted for by the proposal (derived by applying the Load theory of attention, e.g., Lavie, 2005) that ASD is characterized by an increased perceptual capacity (Remington, Swettenham, Campbell, & Coleman, 2009). Recent studies in the visual domain support this proposal. Here we hypothesize that ASD involves an enhanced perceptual capacity that also operates across sensory modalities, and test this prediction, for the first time using a signal detection paradigm. Method: Seventeen neurotypical (NT) and 15 ASD adolescents performed a visual search task under varying levels of visual perceptual load while simultaneously detecting presence/absence of an auditory tone embedded in noise. Results: Detection sensitivity (d′) for the auditory stimulus was similarly high for both groups in the low visual perceptual load condition (e.g., 2 items: p = .391, d = 0.31, 95% confidence interval [CI] [−0.39, 1.00]). However, at a higher level of visual load, auditory d′ reduced for the NT group but not the ASD group, leading to a group difference (p = .002, d = 1.2, 95% CI [0.44, 1.96]). As predicted, when visual perceptual load was highest, both groups then showed a similarly low auditory d′ (p = .9, d = 0.05, 95% CI [−0.65, 0.74]). Conclusions: These findings demonstrate that increased perceptual capacity in ASD operates across modalities.