Joint action refers to the mechanism of co-representing others’ actions when cooperating with them in a shared task, such as the Joint Simon Task. This task requires participants to respond to an assigned stimulus (e.g., diamond) and inhibit responding to another stimulus that is assigned to their partner (e.g., square). When the assigned stimulus is spatially compatible with the response, participants are faster than when stimulus and response are spatially incompatible. Crucially, this Joint Simon Effect (JSE) only occurs, when the task is completed together with a partner whose actions are being co-represented but not when participants perform the ask alone. The JSE is sensitive to various manipulations concerning the social relationship and the similarity between participant and partner, making it suitable for measuring constructs such as in-group favoritism but also an agent’s perceived human-likeness. The goal of our experiment is to examine (i) whether the JSE can be reliably found in an online setting where the players and their responses are only represented via simulated key presses, and (ii) whether the perceived human-likeness and similarity-to-self of a robot partner modulates the JSE. There will be three conditions in the experiment (no partner vs. mechanistic robot vs. humanoid robot; manipulated between participants) and 120 participants will be collected (based on a power analysis assuming a medium effect size). We expect to find no JSE in the no partner condition, as well as a larger JSE in the humanoid versus mechanistic robot condition.