The sensory consequences of intentional actions are perceived to be attenuated compared to equivalent but externally generated stimuli. It is thought that forward models in the sensorimotor system partially cancel predictable reafferent sensory feedback in order to bias attention towards more novel or unexpected stimuli. But does merely observing familiar actions also trigger forward models with attendant sensory attenuation? Previous studies investigated this question in the auditory modality with conflicting results. We conducted two attenuation experiments in a visual modality to conceptually replicate and generalize previous findings (Exp 1), and to control for differences in temporal predictability and attention which may have confounded previous studies (Exp 2). We found that movements initiated by humans (self or other) were attenuated compared to computer movements, and self-initiated movements were the most attenuated. Adding Go signals prior to movements counteracted attenuation. Perceived speed is thus influenced by agency as well as attention.