Objectives: This study aimed to evaluate the effect of progressive muscle relaxation exercises (PMRE) on sleep quality in patients undergoing chemotherapy treatment and experiencing disturbed sleep. Methods: The prospective randomized controlled study was conducted between March and September 2022 with 69 patients (intervention group: 34 patients, control group: 35 patients) in a hospital chemotherapy unit. During the data collection process, the “Personal Information Form” and “Pittsburgh Sleep Quality Index (PSQI)” were utilized. Patients in the intervention group performed PMRE twice a day for 8 weeks. Patients in the control group received routine care at the clinic without additional intervention. For data analysis, Student's t-test, Mann–Whitney U test, Fisher's exact test, and chi-square test were used. Results: The sociodemographic attributes of patients within both the intervention and control groups exhibited comparability. However, notable distinctions emerged in the PSQI Global sleep score and PSQI subdimension scores, encompassing sleep latency and duration, subjective sleep quality, habitual sleep efficiency, sleep disturbance, and daytime dysfunction between the two groups. The study found a notable difference in scores between the patients in the intervention group and those in the control group. The patients who received the intervention had significantly lower scores (P <.001). Conclusion: The study revealed that PMRE was beneficial in improving sleep quality in cancer patients undergoing chemotherapy who had poor sleep quality. Implications for Nursing Practice: Oncology nurses may consider using PMRE to improve the sleep quality of cancer patients receiving chemotherapy.