There is generally a dearth of information involving reference ranges of health variables for numerous elasmobranch species commonly housed in zoos and aquaria; thus, extrapolation from a few existing elasmobranch studies is commonly used to assess health parameters in these species. The primary objective of this study was to establish baseline complete blood cell count and plasma chemistry reference ranges for captive individuals of the cownose ray Rhinoptera bonasus, an elasmobranch that is widely displayed in zoos and aquaria worldwide. This study was conducted using 18 adult cownose rays currently housed at the Mystic Aquarium and Institute for Exploration in Connecticut. Median blood and plasma values were 550 cells/microL for total white blood cell count; 511,250 cells/microL for total red blood cell count; 31% for packed cell volume; 5.8 g/L for total solids; 2.85 g/dL for total protein; 33 units (U)/L for aspartate aminotransferase; 34 U/L for alkaline phosphatase; 0.2 mg/dL for total bilirubin; 1,155 mg/dL for urea nitrogen; 0.1 mg/dL for creatinine; 0.6 g/dL for albumin; 2.15 g/dL for globulins; 144 mg/dL for cholesterol; 157 mg/dL for triglycerides; 45 mg/dL for glucose; 16.9 mg/dL for calcium; 5.8 mg/dL for phosphorus; 294 mmol/L for sodium; 1.55 mmol/L for potassium; and 270 mmol/L for chloride. Gamma glutamyl transpeptidase, amylase, creatine kinase, and lactate dehydrogenase were below the detectable limits for several samples. In nine samples, alanine aminotransferase levels were below the instrument range (< 4 U/L). No significant differences between sexes were detected. The reference ranges reported in this study should provide a useful guide for routine health monitoring of captive cownose rays.