Identifying individual differences in various cognitive constructs has important implications for settings such as clinical and educational assessments. Yet, cognitive tasks themselves often do not measure a single construct. As such, there is value in understanding the mixture of constructs involved in cognitive task performance, as this would allow target constructs to be better isolated. For example, tasks assessing visuospatial working memory (VSWM), in addition to measuring the “of interest” processes of encoding and maintenance, also involve lower-level perceptual processes. Here, we examined perceptual contaminant effects on VSWM using a modified Corsi Block Tapping Task, in which participants observed and reproduced sequences of spatial locations. Two forms of perceptual interference were manipulated: path clutterness, in which distractors appeared near the virtual path connecting successive targets, and endpoint crowding, in which distractors appeared near the target locations themselves. Endpoint crowding produced clear decrements in task performance, whereas path clutterness produced no detectable changes in performance. These findings indicate that perceptual processes, specifically visual crowding, influence VSWM assessment. This underscores the importance of accounting for perceptual processes in the design and interpretation of working memory measures.