The Memory for Intentions Screening Test (MIST) is a clinical measure of prospective memory. There is emerging support for the sensitivity and ecological relevance of the MIST in clinical populations. In the present study, the construct validity of the MIST was evaluated in 40 younger (18-30 years), 24 young-old (60-69 years), and 37 old-old (70+ years) healthy adults. Consistent with expectations derived from the prospective memory and aging literature, older adults demonstrated lower scores on the MIST’s primary scale scores (particularly on the time-based scale), but slightly better performance on the seminaturalistic 24-hour trial. Among the healthy older adults, the MIST showed evidence of both convergent (e.g., verbal fluency) and divergent (e.g., visuoperception) correlations with standard clinical tests, although the magnitude of those correlations were comparable across the time- and event-based scales. Together, these results support the discriminant and convergent validity of the MIST as a measure of prospective memory in healthy older adults.