Background: Breast cancer is a type of cancer with a high incidence in Indonesia. Symptoms of breast cancer include changes in nipple shape, skin depressions, tenderness. One of the treatments is mastectomy, which is the removal of all breast tissue. Postoperatively, mastectomy patients generally experience pain, which can be managed pharmacologically (eg with analgesics) or non-pharmacologically, such as progressive muscle relaxation therapy (PMR). Purpose: To evaluate the effect of the ROP technique in reducing pain levels in left breast cancer patients after mastectomy at the Hospital. Method: A case study on one research subject was conducted using Evidence-Based Nursing (EBN) as the basis for intervention. ROP nursing interventions were performed once daily for three days, each lasting 20–30 minutes, involving seven muscle groups, which were tensed for 5–10 seconds and relaxed for 10 seconds, accompanied by deep breathing techniques. Pain was measured before and after therapy using the Numeric Rating Scale (NRS). Results: There was a decrease in the pain scale from 4 to 1, which indicates that the ROP technique is effective in reducing pain in post-mastectomy patients. Conclusion: The ROP technique has been proven effective in reducing pain levels in post-operative breast cancer patients. Keywords: Breast cancer; Muscle Relaxation; Pain; Progressive. Pendahuluan: Kanker payudara merupakan jenis kanker dengan angka kejadian tinggi di Indonesia. Gejala kanker payudara meliputi perubahan bentuk puting, cekungan kulit, nyeri tekan. Salah satu penanganannya adalah mastektomi, yaitu pengangkatan seluruh jaringan payudara. Pasca Operasi pasien mastektomi umumnya mengalami nyeri, yang dapat dikelola secara farmakologis (misalnya dengan analgesik) maupun non-farmakologis, seperti terapi relaksasi otot progresif (ROP). Tujuan: Untuk mengevaluasi pengaruh teknik ROP dalam menurunkan tingkat nyeri pada pasien kanker payudara sinistra pasca mastektomi di Rumah Sakit Metode: Studi kasus pada satu subjek penelitian dilakukan dengan Implementasi Evidence Based Nursing (EBN) sebagai dasar Intervensi. Tindakan keperawatan ROP dilakukan satu kali per hari selama tiga hari, masing-masing selama 20–30 menit, dengan melibatkan tujuh kelompok otot yang dikencangkan selama 5–10 detik dan direlaksasikan selama 10 detik, disertai teknik pernapasan dalam. Pengukuran nyeri dilakukan sebelum dan sesudah terapi menggunakan Numeric Rating Scale (NRS). Hasil: Terdapat penurunan skala nyeri dari 4 menjadi 1, yang menandakan bahwa teknik ROP efektif dalam menurunkan nyeri pada pasien pasca mastektomi. Simpulan: Teknik ROP terbukti efektif untuk menurunkan skala nyeri pada pasien kanker payudara pasca operasi. Kata Kunci: Kanker payudara; Nyeri; Relaksasi otot progresif.