PURPOSE We previously reported on the pilot study assessing the feasibility of using the Japanese translation of the Comprehensive Score for Financial Toxicity (COST) tool to measure financial toxicity (FT) among Japanese patients with cancer. In this study, we report the results of the prospective survey assessing FT in Japanese patients with cancer using the same tool. PATIENTS AND METHODS Eligible patients were receiving chemotherapy for a solid tumor for at least 2 months. In addition to the COST survey, socioeconomic characteristics were collected by using a questionnaire and medical records. RESULTS Of the 191 patients approached, 156 (82%) responded to the questionnaire. Primary tumor sites were colorectal (n = 77; 49%), gastric (n = 39; 25%), esophageal (n = 16; 10%), thyroid (n = 9; 6%), head and neck (n = 4; 3%), and other (n = 11; 7%). Median COST score was 21 (range, 0 to 41; mean ± standard deviation, 12.1 ± 8.45), with lower COST scores indicating more severe FT. On multivariable analyses using linear regression, older age (β, 0.15 per year; 95% CI, 0.02 to 0.28; P = .02) and higher household savings (β, 8.24 per ¥15 million; 95% CI, 4.06 to 12.42; P < .001) were positively associated with COST score; nonregular employment (β, −5.37; 95% CI, −10.16 to −0.57; P = .03), retirement because of cancer (β, −5.42; 95% CI, −8.62 to −1.37; P = .009), and use of strategies to cope with the cost of cancer care (β, −5.09; 95% CI, −7.87 to −2.30; P < .001) were negatively associated with COST score. CONCLUSION Using the Japanese version of the COST tool, we identified various factors associated with FT in Japanese patients with cancer. These findings will have important implications for cancer policy planning in Japan.