BackgroundObesity is a public health concern with an increasing occurrence worldwide. Literature regarding impact of obesity on results after management of peritoneal carcinomatosis is poor. Our aim was to compare postoperative and oncological outcomes after cytoreductive surgery and hyperthermic intraperitoneal chemotherapy for rare peritoneal malignancies according to the body mass index. MethodsAll the patients managed by cytoreductive surgery and hyperthermic intraperitoneal chemotherapy for rare peritoneal malignancies (including mainly pseudomyxoma peritonei and peritoneal mesothelioma), between 1995 and 2020, were retrospectively included from the French national registry of rare peritoneal tumors. Results1450 patients were retrospectively included (63.5 % female, mean age 54 ± 13 years). Patients were divided into two groups according to their body mass index: non-obese (n = 1248, 86 %) and obese (n = 202, 14 %). Overall morbidity was significantly lower in non-obese patients in comparison with obese patients (n = 532/1248, 43 % vs n = 106/202, 53 %, p = 0.009). Medical and surgical morbidities were significantly lower in non-obese patients in comparison with obese patients (423/1258, 34 % vs n = 86/202, 43 %, p = 0.02 and n = 321/1248, 26 % vs n = 67/202, 33 %, p = 0.003, respectively). One-, 5- and 10-year overall survivals were similar between non-obese and obese patients (95 %, 82 % and 70 % vs 94 %, 76 % and 63 %; p = 0.1). One-, 5- and 10-year disease free survivals were similar between non-obese and obese patients (84 %, 67 % and 61 % vs 79 %, 62 % and 56 %, p = 0.1). ConclusionObese patients have to be carefully managed after cytoreductive surgery and hyperthermic intraperitoneal chemotherapy for rare peritoneal malignancies. Some perioperative prophylactic treatments could be specifically implemented to reduce thromboembolic events, metabolic and wound complications.