OBJECTIVE. This study examined the parallel-forms reliability of a web application (app) of the Activity Card Sort Australia for adults ages 18–64 and assessed its clinical utility. METHOD. Forty-eight participants completed the app and card versions of the tool within a 2- to 3-wk interval and provided feedback via a purpose-designed survey. Intraclass correlation analysis tested parallel-forms reliability. RESULTS. The app demonstrated acceptable parallel-forms reliability for overall retained activity level (r = .75, p < .001), the daily life domain (r = .77, p < .001), and the recreation and relaxation domain (r = .74, p < .001), but not for the physical activity domain (r = .59, p < .001). Clinical utility responses suggested good acceptance of both versions. CONCLUSION. The results suggest that further studies are required before the app version can be used for research or in clinical settings.