The subthalamic nucleus (STN) has been shown to be implicated in the control of voluntary action, especially during tasks involving conflicting choice alternatives or rapid response suppression. However, the precise role of the STN during nonmotor functions remains controversial. First, we tested whether functionally distinct neuronal populations support different executive control functions (such as inhibitory control or error monitoring) even within a single subterritory of the STN. We used microelectrode recordings during deep brain stimulation surgery to study extracellular activity of the putative associative-limbic part of the STN while patients with severe obsessive-compulsive disorder performed a stop-signal task. Second, 2–4 days after the surgery, local field potential recordings of STN were used to test the hypothesis that STN oscillations may also reflect executive control signals. Extracellular recordings revealed three functionally distinct neuronal populations: the first one fired selectively before and during motor responses, the second one selectively increased their firing rate during successful inhibitory control, and the last one fired selectively during error monitoring. Furthermore, we found that beta band activity (15–35 Hz) rapidly increased during correct and incorrect behavioral stopping. Taken together, our results provide critical electrophysiological support for the hypothesized role of the STN in the integration of motor and cognitive-executive control functions.