It is unknown whether nutritional status influences pain intensity in cancer patients receiving a transdermal fentanyl patch (FP). This study aimed to determine whether nutritional status is associated with pain intensity and to evaluate the influence of changes in nutritional status on pain intensity in cancer patients receiving transdermal FP treatment. We included 92 patients receiving transdermal FP treatment for the first time with switching from oxycodone. The patients were classified into low- and normal-nutrition groups based on their nutritional status, which was assessed according to the Nutrition Risk Screening 2002 (NRS 2002) parameters. The pain intensity of each patient was evaluated by a numeric rating scale (11-point scale from 0 to 10). NRS 2002 score and pain intensity were obtained on day 3 after the FP was applied to the skin. Pain intensities were significantly higher among patients in the low-nutrition group than among patients in the normal-nutrition group. NRS 2002 scores showed a significant positive correlation with the pain intensities. In 52 of 92 patients, who were evaluated using the NRS 2002 score and pain intensity on day 30 after FP application, the changes in NRS 2002 scores were significantly related to changes in pain intensities (odds ratio, 30.0; 95% confidence interval, 4.48-200.97; p=0.0005). These results suggest that an increase in the NRS 2002 score is a risk factor for an increase in pain intensity in cancer patients receiving FP treatment. Malnutrition may lead to poor pain management in cancer patients receiving FP treatment.