Performance monitoring is an essential prerequisite of successful goal-directed behavior. Research of the last two decades implicates the anterior midcingulate cortex (aMCC) in the human medial frontal cortex and frontostriatal basal ganglia circuits in this function. Here, we addressed the function of the thalamus in detecting errors and adjusting behavior accordingly. Using diffusion-based tractography, we found that, among the thalamic nuclei, the ventral anterior (VA) and ventral lateral anterior (VLa) nuclei have the relatively strongest connectivity with the aMCC. Patients with focal thalamic lesions showed diminished error-related negativity, behavioral error detection, and posterror adjustments. When the lesions specifically affected the thalamic VA/VLa nuclei, these effects were significantly pronounced, which was reflected by the complete absence of the error-related negativity. These results reveal that the thalamus, particularly its VA/VLa region, is a necessary constituent of the performance-monitoring network, anatomically well connected and functionally closely interacting with the aMCC.