This study investigated the relationship between BMI and patient outcomes after elective primary shoulder replacement based on data from the United Kingdom and Denmark. There were 15,320 and 5,446 operations, mean age was 72.2 years in the UK and 70.5 years in Denmark, mean BMI was 29.4 kg/m² and 28.0 kg/m². Obese patients (BMI 40 kg/m²) had a lower risk of mortality within 365 days after surgery (hazard ratio 0.40). Underweight patients (BMI < 18.5 kg/m²) had a higher risk of mortality within 90 days (hazard ratio 1.69), serious adverse events (odds ratio 1.36), and revision surgery (hazard ratio 1.70). Increasing BMI was not associated with worsening of secondary outcomes. The operation is also safe for obese patients, access to shoulder replacement should not be limited based on BMI alone. Underweight patients are more at risk of mortality, complications and revision surgeries.