The popularity of functional ingredients such as probiotics and postbiotics has increased as pet owners seek ways to improve the health quality and longevity of their pets. Limited research has been conducted regarding the use of probiotics and postbiotics and their effects on canine health. The objective of this study was to evaluate the effects of daily supplementation of Bifidobacterium animalis subsp. lactis CECT 8145, in both live probiotic (PRO) and heat-treated postbiotic (POST) forms, on fecal fermentative end-products and microbiome, insulin sensitivity, serum gut hormones, oxidative stress, inflammatory biomarkers, and white blood cell gene expression of adult dogs. Eighteen adult beagles and 18 adult English pointers were used in a double-blinded placebo-controlled parallel group design, with 12 animals per group (6 English pointers and 6 beagles). The study began with a 60 d adaptation period followed by a 90 d period of daily supplementation with either PRO, POST, or placebo (maltodextrin; CON). Longitudinal assessment of body weight, body condition score, and pelvic circumference did not differ among dietary supplements (P > 0.05). Throughout the experimental period, fecal scores did not differ (P > 0.05); however, fecal pH was lower (P = 0.0049) in the dogs fed POST compared with CON. A higher fecal concentration of propionate (P = 0.043) was observed in dogs fed PRO and POST when compared with CON. While PRO and POST supplementation were associated with changes in bacterial composition at the family and genus level, the overall richness and diversity of the microbiome were not significantly affected. Functional analysis of the metagenome also suggests that PRO and POST supplementation induced potentially beneficial changes in the abundance of pathways involved in pathogenicity, amino acid biosynthesis, and DNA repair. No differences in glycemic or insulinemic responses were observed among the groups (P > 0.05). Dogs supplemented with PRO had a higher (P < 0.05) mean white blood cell leptin relative fold gene expression compared with groups POST and CON. Serum metabolites and complete blood cell counts were within normal ranges and all dogs remained healthy throughout the study. Together, these data suggest that the PRO and POST can safely be supplemented for dogs. Moreover, the results of this study support further investigation of the role of PRO and POST in supporting parameters related to gut health and hormonal regulation.