Purpose: Seroma formation is a common complication in breast cancer patients undergoing mastectomy, and it negatively affects patient recovery after surgery. The present study aimed to evaluate a simple method using fascia suture technique to fix the flap and reduce the incidence of seroma. Methods: A single-center, prospective, randomized controlled trial was carried out among 160 patients who had undergone mastectomy from May 2018 to September 2019. All patients were randomly divided into the fascia suture group (n = 80) or control group (n = 80) and were followed up for at least 3 months for the assessment of immediate and late complications after surgery. Results: No significant differences were observed between the 2 groups with regard to the basic characteristics. Duration of surgery in the fascia suture group was longer by about 6 minutes compared with that in the control group (114.93 ± 13.67 minutes vs. 108.81 ± 15.20 minutes, p = 0.008). The fascia suture group had a shorter duration of drain placement (10.99 ± 3.26 days vs. 13.85 ± 5.37 days, p < 0.001), a smaller volume of the total drainage (460.95 ± 242.92 mL vs. 574.83 ± 285.23 mL, p = 0.007), and the first 3-day drainage (224.96 ± 101.01 mL vs. 272.3 ± 115.47 mL, p = 0.006), compared with the control group. The incidence of seroma formation (G2 or G3) was significantly lower in the fascia suture group compared with the control group (28.8% vs. 12.5%, p = 0.033). Besides, there was no statistical difference between the 2 groups in the assessment of other complications, including postoperative pain, hematoma, surgical site infections, flap necrosis, and skin dimpling (all p > 0.050). Conclusion The fascia suture technique is a simple and effective method for reducing seroma formation and should be used to prevent seroma formation after mastectomy.