Objective: Physical activity (PA), especially moderate-to-vigorous intensity, could protect older adults from cognitive impairment. However, most literature is based on self-reported PA which is limited by recall bias. Light PA is popular among older adults, but a paucity of objective longitudinal data has considered the relationship between light PA and cognitive ability. We examined if a higher level of objectively measured light PA, independent of moderate-to-vigorous physical activity (MVPA), was prospectively associated with better cognitive ability in older adults. Methods: A longitudinal study over 22.12 (+/- 1.46) months including 274 community-dwelling older adults across 14 regions in Taiwan was undertaken. Cognitive ability was obtained using a Chinese version of the Ascertain Dementia 8-item Questionnaire (AD8) and light PA and MVPA captured by 7 days accelerometer positioned on waist. Multivariable negative binomial regression adjusted for confounders were undertaken. Results: 274 participants (74.52 years, 45.6% male) attended the follow-up (96.1%). Higher light PA, independent from MVPA, was associated with a reduced rate of decline in cognitive ability (rate ratio 0.75 [0.60-0.92]). MVPA, was also associated with a reduced decline in cognitive ability (rate ratio 0.85 [0.75-0.95]). Light PA was protective of cognitive ability in sensitivity analyses removing participants with activities of daily living difficulties, depressive symptoms and cognitive impairment at baseline. Conclusion: Our data suggest that light PA may offer a protective influence of future cognitive ability in community dwelling older adults. The promotion of light PA may be a valuable means to maintain cognitive ability in older age. (C) 2017 Elsevier Inc. All rights reserved.