Cognitive deficits are pervasive in youth with Down syndrome (DS) and are likely due to abnormal brain development. Alterations to the prefrontal cortex in particular suggest sustained attention may be a deficit in DS, though some psychological studies found youth with DS performed at developmental level on sustained attention tasks. The current study investigated sustained attention in DS by comparing youth with DS to typically developing (TD) youth matched for cognitive ability. Groups completed visual and auditory sustained attention to response tests (SARTs), as well as tasks to determine if sustained attention predicted their short-term memory. Results indicated groups performed similarly on both SARTs, and sustained attention predicted only a small portion of variance in their short-term memory beyond the influence of cognitive ability. Overall, these findings suggested sustained attention matches developmental level in DS, and it does not predict the pattern of poor auditory relative to visual processing characteristic of DS.