We studied whether objectively measured physical activity (PA) and sedentary behavior (SB) are associated with cognition in Finnish elderly twins. This cross-sectional study comprised twins born in Finland from 1940 to 1944 in the Older Finnish Twin Cohort (mean age, 72.9 years; 726 persons). From 2014 to 2016, cognition was assessed with a validated telephonic interview, whereas PA was measured with a waist-worn accelerometer. In between-family models, SB and light physical activity had significant linear associations with cognition after adjusting for age, sex, wearing time, education level, body mass index, and living condition (SB: β-estimate, −0.21 95% confidence intervals, −0.42 to −0.003; light physical activity: β-estimate, 0.30 95% confidence intervals, 0.02–0.58). In within-family models, there were no significant linear associations between objectively measured PA and cognition. Objectively measured light physical activity and SB are associated with cognition in Finnish twins in their seventies, but the associations were attributable to genetic and environmental selection. •Objectively measured physical activity was associated with cognition in elderly.•Light physical activity was associated with better cognition in individuals.•Sedentary behavior was associated with worse cognition in individuals.•Twin analyses show similar genetics and environment contributing to the associations.