Abstract Background The association of gut microbiota with cancer etiology and prognosis has been demonstrated in humans and rodents but has not been studied in dogs with different types of tumors. Hypothesis/Objectives To analyze microbiome composition according to tumor progression based on metastasis, recurrence, and therapeutic response in canine tumors. Animals Thirty-two client-owned dogs were divided into 3 groups: healthy (n = 9), with lymphoma (n = 12), with nonlymphoid tumors (n = 11). Methods Retrospective case series included animals were divided into subgroups according to the nature and severity of their tumors. Feces were screened for the 16S rRNA gene. Results Overall, alpha diversity was significantly reduced in dogs with tumors (n = 23; 12 lymphoid and 11 nonlymphoid) compared to healthy dogs (n = 9). Bacteroides had lower abundance in canine tumors at genus level. Staphylococcus showed significantly reduced abundance in dogs with aggressive tumor progression. Higher white blood cell (WBC) and neutrophil counts and lower hematocrit were significant in dogs with aggressive tumor. Spearman's rank correlation coefficient analysis revealed several measurements that showed moderate to strong correlations, including Coprococcus with total WBC count, neutrophil count, and hematocrit in the aggressive tumor group, and Saccharimonas with serum albumin and sodium concentration in all tumor dogs. Conclusion and Clinical Importance The diversity of the gut microbiome was significantly reduced in dogs with tumors compared to healthy dogs. Correlations were found between changes in blood measurements and changes in microbiome composition in relation to paraneoplastic syndrome.