Background: Verbal short-term memory (STM) capacity has been considered to support vocabulary learning in typical children and adults, but evidence for this link is inconsistent for studies in individuals with Down syndrome (DS). The aim of this study was explore the role of processing demands on the association between verbal STM and vocabulary measures in DS, by comparing receptive vocabulary measures with high STM processing demands to productive vocabulary measures with low STM processing demands. Method: Forty-seven adults with Down syndrome were administered receptive vocabulary and productive vocabulary tasks, as well as measures of verbal STM abilities and intellectual efficiency. Results: Bayesian regression analyses showed that verbal STM abilities were strongly and specifically associated with receptive vocabulary measures but not productive lexical abilities after controlling for intellectual efficiency, and this is despite the fact that vocabulary abilities as measured by receptive and productive vocabulary tasks were closely associated. Conclusions: In Down syndrome, verbal STM abilities may be predictive of specific task demands associated with receptive vocabulary tasks rather than of vocabulary development per se.