Objective: This study aims to determine the effects of cachexia, causing major problems in the world and Turkey, on self-care agency and quality of life in cancer patients. Methods: The population of this cross-sectional and relationship-seeking study consisted of cancer patients in Turkey from April 1 to April 20, 2021. Using the snowball sampling method, 174 patients were sampled. "Patient Information Form," "The European Organization for Research and Treatment of Cancer C30 Cancer Quality of Life Scale," and "Exercise of Self-Care Agency Scale" were used as data collection tools. Results: In the study, 52 patients (29.9%) were found to have cachexia. Function, general well-being, symptom (except insomnia), and self-care agency, which are subdimensions of the quality-of-life scale, were found to be significantly lower in patients with cachexia than patients without cachexia (P < 0.001). It was determined that there was a significant negative correlation between the cachexia status of the patients and the five basic functions in the functional scale (physical, role, emotional, cognitive, and social function), general well-being, and self-care agency, and there was a significant positive correlation between the cachexia status of the patients and the symptom scale (P < 0.001). According to the results of multiple linear regression analysis, it was found that the factor that significantly affected the cachexia status of the patients was their self-care agency (P < 0.001). Conclusions: It was determined that cachexia caused significantly lower self-care agency and quality of life in cancer patients. Furthermore, quality of life was related to self-care agency.