Background: Hyperglycemia associated with critical illness in nondiabetic human patients is a common occurrence in the intensive care unit (ICU), with a reported incidence as high as 71%. Hypothesis: Hyperglycemia in critically ill dogs increases the risk of morbidity and mortality. Animals: Two hundred forty‐five dogs hospitalized in the ICU over a 2‐month period were evaluated. Methods: Prospective observational study was conducted over a 2‐month period. All dogs in the ICU had their highest daily blood glucose concentration recorded. All dogs with diabetes were excluded from the study. Hyperglycemia was defined as a blood glucose concentration >120 mg/dL. Dogs with hyperglycemia were monitored for persistence and resolution of hyperglycemia. Results: During the study period, 245 dogs were evaluated, of which 38 (16%) were hyperglycemic. Twenty‐six percent (10/ 38) developed hyperglycemia during hospitalization, whereas 74% (28/38) were hyperglycemic at presentation. Length of hospitalization (LOH) was shorter in dogs that presented with hyperglycemia compared with those that developed hyperglycemia during hospitalization (P= .001). Seventy‐one percent (27/38) of dogs were discharged from the hospital, whereas the remaining 29% (11/38) died or were euthanatized. Nonsurvivors had significantly higher median glucose concentration (median, 176 mg/dL; range 122–310 mg/dL) than did survivors (median, 139 mg/dL; 121–191 mg/dL; P= .021). Conclusions and Clinical Importance: The incidence of hyperglycemia in this population of dogs was 16%. Dogs that developed hyperglycemia had longer LOH and nonsurvivors had more pronounced hyperglycemia than did survivors.