Responses to centrally presented target stimuli are faster when they are accompanied by a task-irrelevant lateral accessory stimulus that corresponds spatially with the response hand (accessory variant of the Simon effect). In four experiments, we tested whether this effect depends on the awareness of the accessory stimulus. In a change blindness task, participants were asked to respond to a central letter that was accompanied by a lateral background change on some trials. Change blindness describes the phenomenon that even large changes may remain unnoticed when they occur simultaneously with another visual disruption, e.g., a blank screen. In a series of four experiments, a significant Simon effect was observed both when the accessory stimulus reached awareness and when it remained unnoticed. These results indicate that, based on the spatial location of an accessory stimulus, a spatial code is generated. This code interferes with the response code on the response-selection stage.