Corn protein (CP), a co-product of the corn ethanol industry, is a sustainable protein source used in pet foods. The objectives of this study were to determine the palatability and apparent total tract digestibility (ATTD) of diets containing CP and to test the effects of CP-containing diets on the serum metabolites, hematology, and fecal characteristics, metabolites, and microbiota of healthy adult dogs. Ten female adult beagles (mean age: 6.12 ± 1.39 yr; mean body weight: 9.33 ± 1.04 kg) were used in a replicated 5 × 5 Latin square design (n = 10/treatment). All dietary treatments were based on brewers rice, low-ash chicken byproduct meal, and chicken fat, and contained variable amounts of corn-based proteins: 0% (control), 15.3% corn gluten meal (CGM), 10.2% CGM + 5% CP (Low), 5.1% CGM + 10% CP (Medium), and 15% CP (High). The experiment was composed of five 28-d periods, with each consisting of a 22-d diet consumption period, a 5-d fecal collection period, and 1 d for blood collection. Data were analyzed statistically by Mixed Models using SAS 9.4, with P < 0.05 accepted as being statistically significant. Two 2-d palatability studies (n = 20 dogs) were also conducted to compare the High diet vs. control diet and High diet vs. CGM diet. Dogs were shown to prefer (P < 0.05) the High diet over the control diet by a ratio of 1.8:1, but no preference was observed between the High and CGM diets. In the digestibility study, the ATTD of dry matter, organic matter, and energy increased (P < 0.001) linearly with CP inclusion. The ATTD of fat was greater (P < 0.001) for the control diet than for the CGM, Low, and High diets. Fecal scores were lower (P = 0.05; firmer) and fecal dry matter percentage was higher (P < 0.0001) in dogs fed CGM than those fed CP. Fecal phenol and indole, short-chain fatty acid, and branched-chain fatty acid concentrations were greater (P < 0.05) in dogs fed CP than dogs fed CGM and control. Fecal bacterial diversity was not altered by diet, but the relative abundance of approximately 10 bacterial genera was altered by diet. In summary, our data demonstrate that the inclusion of CP in dog foods resulted in high diet palatability and macronutrient digestibility and altered microbial composition and activity.