Cancer is a substantial global health problem both in humans and animals with a consistent increase in mortality and incidence rate. The commensal microbiota has been involved in the regulation of several physiological and pathological processes, both within the gastrointestinal system and at distant tissue locations. Cancer is not an exception, and different aspects of the microbiome have been described to have anti‐ or pro‐tumour effects. Using new techniques, for example high‐throughput DNA sequencing, microbial populations of the human body have been largely described and, in the last years, studies more focused on companions' animals have emerged. In general, the recent investigations of faecal microbial phylogeny and functional capacity of the canine and feline gut have shown similarities with human gut. In this translational study we will review and summarize the relation between the microbiota and cancer, in humans and companion animals, and compare their resemblance in the type of neoplasms already studied in veterinary medicine: multicentric and intestinal lymphoma, colorectal tumours, nasal neoplasia and mast cell tumours. In the context of One Health, microbiota and microbiome integrative studies may contribute to the understanding of the tumourigenesis process, besides offering an opportunity to develop new diagnostics and therapeutic biomarkers both for veterinary and human oncology.