Introduction: Pain typically impairs task performance, increases fatigue, and is associated with behavioral disengagement. Together, this suggests that pain impacts effort, defined as the mobilization of resources to carry out behavior. However, empirical evidence on this issue is lacking. Objectives: Drawing on motivational intensity theory, this study investigated the impact of pain on effort mobilization during cognitive task performance. Methods: We administered individually adjusted painful thermal stimulations during an easy memory task during which we measured effort as cardiovascular reactivity—a valid and objective measure. Control conditions included painful stimulations alone, the memory task alone, and the memory task with nonpainful heat stimulations. We expected pain to increase subjective difficulty due to additional demand on cognitive functioning, and in turn effort according to motivational intensity theory. Effort-related cardiovascular reactivity was predicted to increase along these conditions: pain-alone, task-alone, task with nonpainful stimulations, and the strongest effort when painful stimulations were administered during the task. Results: Overall, the findings offered support to our hypotheses. As predicted, results showed that pain increased subjective task difficulty. Moreover, most of the effort-related cardiovascularmeasures showed the expected patternwith the strongest reactivity when painful stimulations were administered during the task compared with the other conditions (p , 0.02). Conclusion: These results are first evidence that pain impacts mental effort assessed as cardiovascular reactivity. We discuss how such extra effort might be related to the persistent feeling of fatigue and behavioral disengagement frequently observed in patients with chronic pain.