Background: Infection following implant-based breast reconstruction can lead to devastating complications. Risk factors for infection include smoking, diabetes, and obesity. Intraoperative hypothermia may represent another modifiable risk factor. This study analyzed the effect of hypothermia in postmastectomy immediate implant-based reconstruction on postoperative surgical-site infection (SSI). Methods: This was a retrospective review of 122 patients with intraoperative hypothermia, defined as less than 35.5°C, and 106 normothermic patients who underwent postmastectomy implant-based reconstruction between 2015 and 2021. Demographics, comorbidities, smoking status, hypothermia (and its duration), and length of surgery were collected. The primary outcome was SSI. Secondary outcomes included reoperation and delayed wound healing. Results: A total of 185 patients (81%) underwent staged reconstruction with tissue expander placement and 43 patients (18.9%) had a direct-to-implant procedure. Over half (53%) of the patients experienced intraoperative hypothermia. In the hypothermic group, a higher proportion of patients had SSIs (34.4% versus 17% of normothermic patients; P < 0.05) and wound healing complications (27.9% versus 16%; P < 0.05). Intraoperative hypothermia predicted SSI (OR, 2.567; 95% CI, 1.367 to 4.818; P < 0.05) and delayed wound healing (OR, 2.023; 95% CI, 1.053 to 3.884; P < 0.05). Longer duration of hypothermia significantly correlated with SSI, with an average 103 minutes versus 77 minutes (P < 0.05). Conclusions: This study demonstrates that intraoperative hypothermia is a significant risk factor for postoperative infection in postmastectomy implant-based breast reconstruction. Maintaining strict normothermia during implant-based breast reconstruction procedures may improve patient outcomes by reducing the risk of postoperative infection and delayed wound healing. CLINICAL QUESTION/LEVEL OF EVIDENCE: Risk, II.