Adaptive behavior requires the ability to appropriately react to action errors. Post-error slowing of response times (PES) is one of the most reliable phenomena in human behavior. It has been proposed that PES is partially achieved through inhibition of the motor system. However, there is no direct evidence for this link – or indeed, that the motor system is physiologically inhibited after errors altogether. Here, we used transcranial magnetic stimulation and electromyography to measure cortico-spinal excitability (CSE) across four experiments using a Simon task, in which female and male human participants sometimes committed errors. Errors were followed by reduced CSE at two different time points and in two different modes. Shortly after error commission (250ms) CSE was broadly suppressed – i.e., even task-unrelated motor effectors were inhibited. During the preparation of the subsequent response, CSE was specifically reduced at task-relevant effectors only. This latter effect was directly related to PES,...