This case report describes a rare instance of a 33-year-old woman with systemic lupus erythematosus (SLE) who experienced a pregnancy complicated by preeclampsia, eclampsia, and postpartum atypical hemolytic uremic syndrome (aHUS). At 28 weeks and four days of gestation, the patient presented with severe hypertension, proteinuria, and a loss of consciousness, leading to an emergency cesarean section. Postoperatively, the patient developed acute kidney injury, respiratory failure, and thrombotic microangiopathy (TMA). Although she exhibited the classic triad of hemolytic anemia, thrombocytopenia, and renal dysfunction, normal complement levels ruled out postpartum exacerbation of SLE, and aHUS was not diagnosed during hospitalization. Differential diagnoses, including HELLP (Hemolysis, Elevated Liver Enzyme levels, and Low Platelet levels) syndrome, thrombotic thrombocytopenic purpura, and Shiga toxin-producing Escherichia coli (STEC)-HUS, were excluded. Schistocytes appeared on postoperative day 5, leading to the cessation of tacrolimus and the initiation of prednisolone. Continuous hemodiafiltration and mechanical ventilation facilitated gradual recovery, and the patient was discharged on postoperative day 26. Post-discharge genetic testing revealed no pathogenic mutations; however, the clinical presentation supported a diagnosis of aHUS. aHUS driven by excessive complement activation requires prompt recognition and treatment with plasma exchange or anti-complement monoclonal antibodies (e.g., eculizumab). In this case, delayed recognition of aHUS precluded the use of such therapies. This case highlights the importance for clinicians to consider the possibility of aHUS in postpartum patients with severe renal dysfunction and TMA symptoms, even if the patient has an underlying SLE, as early diagnosis and treatment of aHUS is necessary to improve maternal outcomes.