We used the Danish National Health Registers to conduct a study on the prevalence of obesity-related comorbidities in Danish citizens who have been diagnosed with obesity at a Danish hospital. This was a retrospective observational study with a population comprising all Danish citizens (≥18 years) who have been registered with a specific obesity class diagnosis in the Danish National Patient Register between 2002 and 2018. A total of 86 980 persons with hospital-diagnosed obesity were included in the study population. To investigate how the risk of having comorbidities varies with the degree of obesity, we applied adjusted logistic regression to estimate the odds ratio of having one of the following predefined comorbidities for people with a BMI in obesity classes II and III compared with people with a BMI in obesity class I: type 2 diabetes, ischaemic heart disease, non-alcoholic steatohepatitis and non-alcoholic fatty liver disease, hip and knee osteoarthritis, obstructive sleep apnoea and asthma. Comorbidities were defined from ICD-10 diagnosis codes and prescription medication utilization. The odds ratio for obstructive sleep apnoea (OR 1.86 and OR 3.0), type 2 diabetes (OR 1.68 and OR 2.26), hip and knee osteoarthritis (OR 1.29 and OR 1.54) and asthma (OR1.13 and OR 1.25) increased significantly with obesity class (obesity class II relative to I and III relative to I, respectively). The odds ratio of having had at least one comorbidity was estimated to be 1.52 for people with a BMI in obesity class II and 2.10 for people with a BMI in obesity class III compared with people in obesity class I. The risk of obstructive sleep apnoea, type 2 diabetes, hip and knee osteoarthritis, and asthma increased significantly with increasing BMI, highlighting the importance of preventing further weight gain even in individuals who are already living with obesity.