Background: Standard treatment for locally advanced rectal cancer involves neoadjuvant chemoradiotherapy (NACRT) followed by definitive surgery. Exercise during NACRT may improve outcomes in these patients, however, no study to date has examined their motivation to exercise during NACRT. Purpose: To explore exercise motivation in rectal cancer patients during NACRT. Methods: Patients (N=18) participated in a supervised aerobic exercise program during NACRT. Using the theory of planned behavior, we assessed perceived benefits, enjoyment, support, motivation, and difficulty before and after the intervention. We also assessed patient-reported specific benefits, harms and barriers. Results: Perceived enjoyment (p=0.003) and difficulty (p=0.037) improved from pre-NACRT to post-NACRT. The most frequently cited benefits of the exercise program were improvements in cardiovascular fitness (75 %), quality of life (75%), and self-esteem (65%). The most frequently cited harms of the exercise program were exacerbation of fatigue (31%) and diarrhea (31%). Finally, the most frequently cited barriers to the exercise were side effects of chemoradiotherapy (88%), fatigue (76%), and diarrhea (71%). Conclusions: This pilot study suggests that rectal cancer patients found an aerobic exercise intervention during NACRT to be more enjoyable and less difficult than anticipated. Moreover, rectal cancer patients identified many potential benefits but they also identified potential harms that need to be closely monitored in future interventions. Finally, most of the barriers to the supervised exercise were treatment-related and will need to be addressed if exercise is to be tested and ultimately incorporated into standard care for rectal cancer patients receiving NACRT.