Background Leiomyosarcoma is a rare malignant tumor of smooth muscle origin and represents 10–20% of all soft tissue sarcomas. Primary colon and rectal sarcomas constitute TP53 mutations. Patients with a history of cancer who do not meet all the “classic” criteria for Li–Fraumeni syndrome are considered to have Li–Fraumeni-like syndrome. To the best of our knowledge, this article is the first report of a patient with rectal leiomyosarcoma as the initial phenotypic manifestation of Li–Fraumeni-like syndrome. The authors also present a literature review. Case presentation A 67-year-old Brazilian woman underwent anterior rectosigmoidectomy and panhysterectomy secondary to rectal leiomyosarcoma. She subsequently developed carcinomatosis and died 2 years after the operation. Her family medical history consisted of a daughter who died at 32 years of age from breast cancer, a granddaughter diagnosed with adrenocortical carcinoma at 6 years of age and two siblings who died from prostate cancer. A genetic study was carried out to identify a pathogenic variant of Li–Fraumeni syndrome. In the DNA extracted from the peripheral blood leukocyte, restriction fragment length polymorphism was analyzed to search for mutations in the TP53 gene. The DNA sequencing identified the germline pathogenic variant p. R337H heterozygous in exon 10 of TP53. The patient was classified as having Li–Fraumeni-like syndrome. Conclusion In patients with rectal leiomyosarcoma, it is advisable to investigate the family history of cancer and perform genetic studies to screen for Li–Fraumeni syndrome.