Our study aims to identify the risk factors and dosimetry characteristics associated with capsular contracture. Methods: We retrospectively analyzed 118 women with breast cancer who underwent PMRT following an IBR between 2010 and 2022. Patients were treated with PMRT of 50.0–50.4 Gy in 25–28 fractions. Capsular contracture was categorized according to the Baker Classification for Reconstructed Breasts. Results: After a median follow-up of 22 months, the incidence of clinically relevant capsular contracture (Baker III–IV) was 22.9%. Overall, capsular contracture (Baker I–IV) occurred in 56 patients (47.5%) after a median of 9 months after PMRT. The rate of reconstruction failure/implant loss was 25.4%. In the univariate analysis, postoperative complications (prolonged pain, prolonged wound healing, seroma and swelling) and regional nodal involvement were associated with higher rates of capsular contracture (p = 0.017, OR: 2.5, 95% CI: 1.2–5.3 and p = 0.031, respectively). None of the analyzed dosimetric factors or the implant position were associated with a higher risk for capsular contracture. Conclusion: Postoperative complications and regional nodal involvement were associated with an increased risk of capsular contracture following breast reconstruction and PMRT, while none of the analyzed dosimetric factors were linked to a higher incidence. Additional studies are needed to identify further potential risk factors.