Background The “obesity paradox” reflects an observed relationship between obesity and decreased morbidity and mortality, suggesting improved health outcomes for obese individuals. Studies examining the relationship between high body mass index ( BMI ) and adverse outcomes after cardiac surgery have reported conflicting results. Methods and Results The study population (N=78 762) was comprised of adult patients who had undergone first‐time coronary artery bypass ( CABG ) or combined CABG /aortic valve replacement ( AVR ) surgery from April 1, 1998 to October 31, 2011 in Ontario (data from the Institute for Clinical Evaluative Sciences). Perioperative outcomes and 5‐year mortality among pre‐defined BMI (kg/m 2 ) categories (underweight <20, normal weight 20 to 24.9, overweight 25 to 29.9, obese 30 to 34.9, morbidly obese >34.9) were compared using Bivariate analyses and Cox multivariate regression analysis to investigate multiple confounders on the relationship between BMI and adverse outcomes. A reverse J‐shaped curve was found between BMI and mortality with their respective hazard ratios. Independent of confounding variables, 30‐day, 1‐year, and 5‐year survival rates were highest for the obese group of patients (99.1% [95% Confidence Interval { CI }, 98.9 to 99.2], 97.6% [95% CI , 97.3 to 97.8], and 90.0% [95% CI , 89.5 to 90.5], respectively), and perioperative complications lowest. Underweight and morbidly obese patients had higher mortality and incidence of adverse outcomes. Conclusions Overweight and obese patients had lower mortality and adverse perioperative outcomes after cardiac surgery compared with normal weight, underweight, and morbidly obese patients. The “obesity paradox” was confirmed for overweight and moderately obese patients. This may impact health resource planning, shifting the focus to morbidly obese and underweight patients prior to, during, and after cardiac surgery.