PURPOSE: Breast cancer survivors often experience cancer-related cognitive impairment (CRCI), such as problems with memory and attention. However, typical neuropsychological test batteries are unable to capture the day-to-day variability of cognition and may be underestimating CRCI. The present study aims to assess the feasibility, usability, and validity of a novel ecological momentary assessment (EMA) app of cognition. METHODS: Nineteen breast cancer survivors 6-36-month post-chemotherapy and 28 healthy controls completed the NIH Toolbox Cognition Battery. Subsequently, participants completed the EMA app (once a day, for 30 days) comprising four cognitive tasks assessing processing speed, working memory, inhibition, and attention. At the conclusion of the app, participants completed a usability questionnaire on which content analysis was performed. Feasibility was assessed against eight criteria, including accessibility, app compliance, and technical smoothness. Convergent construct validity was assessed using Spearman's correlation analyses between the NIH toolbox and the EMA app. RESULTS: Five of eight feasibility criteria were met, including accessibility, app motivation, participation rate, drop-out, and data collection. Additionally, our content analyses revealed four themes important to usability: self-development, altruism, engagement, and functionality. Majority of the EMA tasks were moderately positively correlated with the corresponding constructs of the NIH toolbox tasks (R's range 0.55-0.64), indicating better performance on the EMA app coincided with better performance on the NIH toolbox. CONCLUSIONS: Our findings show the app was accessible, participants were motivated to complete sessions, and our tasks showed good construct validity. IMPLICATIONS FOR CANCER SURVIVORS: Our novel EMA app can be used as a comprehensive cognitive measure in cancer survivors.