Background: Significant postoperative discomfort is frequently experienced by patients undergoing modified radical mastectomy (MRM). This study aimed to assess whether administering an ultrasound-guided serratus plane block (SPB) in addition to standard general anesthetia could optimize intraoperative and postoperative analgesia in patients undergoing MRM, as compared to placebo . Methods: In a prospective, double-blind, randomized research, thirty women scheduled for MRM were randomly assigned to receive 30 mL of either normal saline (control, or "GC" group) or 0.25% bupivacaine (serratus, or "GS" group) under ultrasound guidance before conventional general anesthesia was administered. The primary outcome was postoperative pain scores using VAS during movement and at rest. Secondary measures included time to initial rescue analgesia, total 24-hour morphine consumption, hemodynamic stability (heart rate and mean arterial pressure), intraoperative fentanyl use, complications, and mobilization within the first 12 hours. Results: The GS group had significantly lower mean VAS scores at rest and with movement at all measuring times. The GS arm had lower intraoperative fentanyl usage, consumed less morphine over 24 hours, and showed longer first time to rescue analgesia (p<0.001 for all). Compared to 33% of controls, 80% of GS patients mobilized within 12 hours (p=0.02). Those who received SPB had stable hemodynamic parameters (p<0.05). There were no complications that might be linked to the block. Conclusion: When combined with general anesthesia for MRM, ultrasound-guided SPB provides better intra- and postoperative pain control, prolongs the time to first rescue morphine, reduces the need for opioids, and encourages faster mobilization without sacrificing hemodynamic stability.