To evaluate the prevalence of overweight/obesity and associated complications from a large, cross-sectional, nationwide database in China. Data were obtained from 519 Meinian health check-up centres across 243 cities. Eligible participants were aged ≥18 years, with a routine check-up in 2019 (N = 21 771 683) and complete height, weight, sex and region data. The unadjusted prevalence rates of overweight/obesity were calculated by age, sex and region. In addition, the nationwide prevalence rates of overweight and obesity were standardized according to the 2010 China census by age group and sex. The prevalence of obesity-related complications by body mass index (BMI) groups was calculated using logistic regression. There were 15 770 094 eligible participants (median age 40 years; mean BMI 24.1 kg/m2 ; 52.8% male). By Chinese BMI classification, 34.8% were overweight and 14.1% were obese. Overweight and obesity were more prevalent in male than female participants (standardized: overweight 40.2% vs. 27.4%; obesity 17.6% vs. 9.6%, respectively). The prevalence of assessed complications was higher in participants with overweight/obesity versus those with normal BMI (P < 0.001 for trends). The most prevalent complications in participants with overweight/obesity were fatty liver disease, prediabetes, dyslipidaemia and hypertension. The number of complications increased with higher BMI. Overweight/obesity and related complications are highly prevalent in this population. These data may better inform management and prevention public health strategies in China.