Background <p>Adult T-cell leukemia/lymphoma (ATLL) is a rare peripheral T-cell malignancy induced by Human T-cell lymphotropic virus type 1 (HTLV-1) infection, commonly affecting individuals in their 50s to 60s. The disease manifests through various clinical forms, with skin involvement being a frequent presentation. While HTLV-1 has been linked to several cancers, including non-ATLL lymphomas and liver cancer. This case report discusses the rare coexistence of sigmoid colon adenocarcinoma and cutaneous ATLL in a 75-year-old woman.</p> Materials and methods <p>A 75-year-old woman presenting with abdominal pain, vomiting, distention, and constipation underwent clinical evaluation including physical examination, laboratory tests, abdominal imaging, and contrast-enhanced CT. Due to large-bowel obstruction, she received emergency surgical management consisting of left hemicolectomy, splenic flexure mobilization, regional lymphadenectomy, and Hartmann colostomy. Tissue from the resected colon and later cutaneous lesions was submitted for histopathological examination and immunophenotypic profiling with an extended T-cell marker panel. Serologic testing for HTLV-1 was also performed. Additionally, a systematic literature review was conducted according to PRISMA 2020 guidelines. Comprehensive searches of PubMed, Scopus, and Web of Science from database inception to November 3, 2025, used controlled vocabulary and free-text terms related to HTLV-1/ATLL and gastrointestinal malignancies. Case reports and case series involving human subjects were independently screened in two phases.</p> Results <p>The patient was diagnosed with sigmoid colon adenocarcinoma requiring emergency surgery and later developed cutaneous lesions, which were confirmed as smoldering ATLL through histopathology, immunophenotyping (mature CD4⁺/CD25⁺ T-cell phenotype with CD7 loss), and positive HTLV-1 serology. She is receiving adjuvant chemotherapy for colorectal cancer while undergoing surveillance for ATLL. The systematic search identified 365 records; after removal of duplicates and screening, four eligible publications were included. Of these four studies, three were case reports, all describing patients with HTLV-1 infection and gastric adenocarcinoma. The fourth publication was a case series including three patients; two of them ATLL and gastric adenocarcinoma, consistent with the findings of our study, while the third patient had ATLL and colorectal adenocarcinoma. Overall, the reported clinical outcomes varied, underscoring the rarity of this coexistence and the lack of standardized management approaches.</p> Conclusion <p>This case highlights the rare occurrence of sigmoid colon adenocarcinoma and cutaneous ATLL in an HTLV-1-positive patient, emphasizing the potential for HTLV-1 to contribute to multiple malignancies. Our findings underscore the need for further research to explore the link between HTLV1 and gastrointestinal cancers, as well as its broader oncogenic potential in other malignancies. The coexistence of these two diseases presents diagnostic challenges and calls for an integrated approach to patient management in HTLV-1-endemic areas.</p>