We report the case of a 75-year-old female patient with a big tumour in the lower rectum with intestinal obstruction and lower gastrointestinal bleeding history who underwent a tumour biopsy under laparotomy and end colostomy at another hospital in Peru. She came to our institution for clinical evaluation with a pathology result of a rectal gastrointestinal stromal tumour. An extra elevator abdominoperineal resection was performed with tumour-free margins. The histology confirmed a high-grade (G2) rectal gastrointestinal stromal tumour with a mitotic index of 27/50. DOC-1 (+) and CD117 (+) in immunohistochemistry. Genomic DNA was extracted from the paraffin-fixed tumour sample, and c.1504_1509dupGCCTAT (p.Ala502_Tyr503dup) mutation was detected in exon 9 of the KIT gene. Imatinib 400 mg per day for 3 years was indicated as adjuvant treatment. Currently, she has a disease-free survival of 12 months.