Breast reconstruction using tissue expanders and silicone implants has become increasingly popular in Japan since health insurance began covering the procedure in 2013. Radiotherapy after mastectomy is recommended for certain patients and has been identified as a major risk factor for capsular contractures. However, the effect of radiation therapy timing on the development of capsular contracture has not yet been thoroughly studied. This study aimed to evaluate the effects of radiation therapy timing on the development of capsular contractures in patients who underwent implant-based breast reconstruction performed by a single expert plastic surgeon. This retrospective cohort study included 341 patients undergoing implant-based breast reconstruction and irradiation between April 2003 and March 2019. Patients were categorized based on radiation and operation types. Variables included postirradiation skin condition, implant mobility, and expander placement position. The outcome measure was the development of capsular contracture, assessed using the Baker Classification. Of 340 patients, 43 developed severe capsular contracture (Baker classification Grade III) within 2 years (cumulative incidence, 12.6 %). No significant relationship was found between the radiation or operation type and capsular contracture. Instead, postirradiation skin redness, implant mobility, skin pinchability after 1 year, and expander positioning were found to be significant factors affecting capsular contracture development. The timing of radiation therapy was not a determinant of capsular contracture development. Factors such as postirradiation skin inflammation, implant mobility, ability to pinch the skin, and expander position play pivotal roles in determining capsular contracture development.