Short-term memory (STM) challenges are often observed in children with Down syndrome (DS), but existing early STM measures introduce measurement confounds in this population. To address the need for valid early STM measures for future DS interventions, this study evaluated the psychometric properties of a modified Corsi Span task, administered to 110 children with DS. Results indicated that the modified Corsi task has feasibility in the age range of 5-8 years and is scalable across chronological and mental ages. Minimal practice effects and evidence of test-retest reliability and convergent validity were observed. Implications for using a modified Corsi task in studies of early STM and treatment trials for children with DS are discussed.