Non-pharmacological therapies, such as physical activity interventions, are an appealing alternative or add-on to current pharmacological treatment of cognitive symptoms in patients with dementia. In this meta-analysis, we investigated the effect of physical activity interventions on cognitive function in dementia patients, by synthesizing data from 802 patients included in 18 randomized control trials that applied a physical activity intervention with cognitive function as an outcome measure. Post-intervention standardized mean difference (SMD) scores were computed for each study, and combined into pooled effect sizes using random effects meta-analysis. The primary analysis yielded a positive overall effect of physical activity interventions on cognitive function (SMD[95% confidence interval] = 0.42[0.23;0.62], p < .01). Secondary analyses revealed that physical activity interventions were equally beneficial in patients with Alzheimer's disease (AD, SMD = 0.38[0.09;0.66], p < .01) and in patients with AD or a non-AD dementia diagnosis (SMD = 0.47[0.14;0.80], p < .01). Combined (i.e. aerobic and non-aerobic) exercise interventions (SMD = 0.59[0.32;0.86], p < .01) and aerobic-only exercise interventions (SMD = 0.41[0.05;0.76], p < .05) had a positive effect on cognition, while this association was absent for non-aerobic exercise interventions (SMD = -0.10[-0.38;0.19], p = .51). Finally, we found that interventions offered at both high frequency (SMD = 0.33[0.03;0.63], p < .05) and at low frequency (SMD = 0.64[0.39;0.89], p < .01) had a positive effect on cognitive function. This meta-analysis suggests that physical activity interventions positively influence cognitive function in patients with dementia. This beneficial effect was independent of the clinical diagnosis and the frequency of the intervention, and was driven by interventions that included aerobic exercise.