The serratus posterior superior intercostal plane (SPSIP) block is a novel technique recently described for thoracic analgesia. This study presents two cases using this technique for postoperative pain after mastectomy with axillary lymph node dissection. The SPSIP block was administered to the patients in the preoperative period as part of multimodal analgesia, and postoperative pain was monitored using the numeric rating scale (NRS). In both patients, the NRS pain scores were below 3/10. SPSIP provided adequate postoperative analgesia in these cases without the need for any opioid agents. Thus, an SPSIP block can be a valuable treatment option for postoperative pain after breast surgery.