Tomato products, good sources of lycopene, may lower the incidence of prostate cancer, but data on the effectiveness of lycopene supplementation during radiation therapy are lacking. This study aimed to evaluate the tolerance and acceptance of three different amounts (4, 8, or 12 oz) of tomato juice (TJ) and their effect on serum lycopene during radiotherapy in 20 men with localized prostate cancer. Participants were randomized into a control group or one of three intervention groups who consumed TJ daily during treatment. Dietary lycopene intake was estimated using the National Cancer Institute (NCI) Diet History Questionnaire, and gastrointestinal tolerance of TJ was evaluated using the NCI Cancer Therapy Evaluation Program: Common Toxicity Criteria v 2.0. Serum and TJ lycopene levels were measured by liquid chromatography-mass spectrometry. TJ was well tolerated without any gastrointestinal side effects, and increased serum lycopene levels were observed in the 8 and 12 oz groups from baseline to endpoint. No correlation between serum and dietary lycopene was detected. Despite no reported change in dietary intake, non-significant weight loss was observed in the control group but not the intervention group participants. A significant positive correlation between serum lycopene, weight, and body mass index, and a negative correlation between serum lycopene and piror nutritional supplement use was detected. Weight change should be monitored and evaluated during treatment. Larger clinical trials are needed to validate the use of TJ to increase serum/dietary lycopene intake and correlate with side effects during radiotherapy in men with prostate cancer.