Background: Chronic rectal toxicity significantly decreases the quality of life for men who receive radiotherapy for prostate cancer. The volume of rectum exposed to 70 Gy or more is a validated metric that predicts the risk of late rectal toxicity. We hypothesized that MRI as compared to CT-based prostate radiotherapy treatment planning can reduce the volume of rectum exceeding 70 Gy. Methods: This prospective study single arm study enrolled 15 men treated with external beam radiation therapy for localized prostate cancer. At the time of treatment planning a 3 Tesla T2-weighted magnetic resonance imaging examination of the prostate was obtained. A radiotherapy plan was designed by a medical physicist using identical constraints for both CT and MRI-based consensus volumes defined by a radiologist and radiation oncologist. The volume of rectum exposed to 70 Gy or more was calculated and compared using the paired Signed Rank Test. Results: The median age was 70 years (range 56-84), median PSA 7.3 ng/mL (range 3.2 – 22.1), and median prostate volume 40 mL (range 25 - 65) by transrectal ultrasound. Sixty percent (n=9) were intermediate risk and 40 percent (n=6) high risk by NCCN guidelines. The majority were either clinical stage T1c (n=7) or T2 (n=6). Two men had extracapsular extension (T3a). None of the participants had seminal vesicle invasion (T3b), rectal or bladder involvement (T4), or lymph node metastasis (N1). All 15 men enrolled on the study completed both a standard radiation planning CT and research MRI examination. For CT-based treatment plans the median volume of rectum receiving 70 Gy or more was 9.3 cubic centimeters (cc) [IQR 7.0 to 10.2] compared with 4.9 cc [IQR 4.1 to 8.7] for MRI-based plans. This resulted in a median volume reduction of 2.1 cc [IQR 0.5 to 5.3, P< .001]. Conclusions: MRI-planned prostate radiotherapy can reduce the volume of rectum receiving radiation doses in excess of tolerance (70 Gy or more) and should be considered in men who are at high risk for late rectal toxicity. Clinical trial information: NCT02470910