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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 cognitive neuroscience. 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 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-related effectors only. This latter effect was directly related to PES, with stronger CSE suppression accompanying greater PES. This suggests that PES is achieved through increased inhibitory control during post-error responses. To provide converging evidence, we then re-analyzed an openly-available EEG dataset that contained both Simon- and Stop-signal tasks using independent component analysis. We found that the same neural source component that indexed action-cancellation in the stop-signal task also showed clear PES-related activity during post-error responses in the Simon task. Together, these findings provide clear evidence that post-error adaptation is partially achieved through motor inhibition. Moreover, inhibition is engaged in two modes (first non-selective, then selective), aligning with recent multi-stage theories of error processing.