Human observers combine multiple sensory cues synergistically to achieve greater perceptual sensitivity, but little is known about the underlying neuronal mechanisms. We recorded from neurons in the dorsal medial superior temporal area (MSTd) during a task in which trained monkeys combine visual and vestibular cues near optimally to discriminate heading. During bimodal stimulation, MSTd neurons combine visual and vestibular inputs linearly with sub-additive weights. Neurons with congruent heading preferences for visual and vestibular stimuli show improvements in sensitivity that parallel behavioral effects. In contrast, neurons with opposite preferences show diminished sensitivity under cue combination. Responses of congruent cells are more strongly correlated with monkeys' perceptual decisions than opposite cells, suggesting that the animal monitors the activity of congruent cells to a greater extent during cue integration. These findings demonstrate perceptual cue integration in non-human primates and identify a population of neurons that may form its neural basis.