Increasing the magnitude of an offered reward for an action can improve performance. However, performance can suffer when the payoff is exceptionally large, a phenomenon dubbed “choking under pressure.” What is the neural basis of choking under pressure, and what are the neural mechanisms whereby potential rewards affect motor performance? To study the interaction between rewards and movement, we recorded neural population activity in the motor cortex of rhesus monkeys as they performed a challenging delayed-reaching task, in which the reward available for a successful reach was pre-cued. Both animals choked under pressure, showing an “inverted-U” relationship between success rate and reward size. How does the brain mediate this unintuitive relationship between monotonic reward information and an inverted-U trend in behavioral success? We examined how reward size altered the neural activity before movement onset for different reach directions. We identified three main effects: First, increasing reward yielded a monotonic shift in neural activity along a “reward axis.” Second, we found that this reward axis was nearly orthogonal to the linear 2D projection that best separates neural activity for reach directions (“target plane”). Third, increasing rewards initially drove average preparatory activity for different reach directions farther apart in the target plane, but the highest reward pushed them back together. This expansion-then-contraction of target conditions was sufficient to yield an inverted-U in the accuracy of an offline discriminator of the reach direction as a function of reward. In summary, increasing reward initially drives motor cortical preparatory activity in a manner that is more informative about the upcoming movement (as measured by decoding accuracy), but then as reward signals grow, less information about target location is present in the population response. In this way, a neural correlate of choking under pressure is evident in the motor cortex.