BACKGROUND: The association of long-term weight change with atrial fibrillation (AF) risk remains controversial and evidence for the effect of metabolic health transition on AF is limited. OBJECTIVE: To evaluate the effects of body mass index (BMI) change and transition in BMI-metabolic health status on AF. METHODS: We conducted a prospective cohort study within UK Biobank. Metabolic health was defined as having at least four of the six metabolically healthy (MH) criteria including blood pressure, C-reactive protein, triacylglycerols, low-density lipoprotein cholesterol, high-density lipoprotein cholesterol, and glycated hemoglobin. RESULTS: Among 490,969 participants initially included for baseline BMI analysis, a total of 33,297 AF cases were observed during a median follow-up of 14.0 years. Overweight (hazard ratio [HR] 1.12, 95% confidence interval [CI] 1.09-1.15) and obesity (1.74, 1.68-1.79) significantly increased the risk of AF. A BMI loss of over 2% per year was associated with a lower risk of AF (0.75, 0.57-0.99), especially transitioning from obesity to overweight (0.74, 0.54-1.02), though without statistical significance. Compared with MH-normal weight, the HRs for MH-obesity and metabolic unhealthy (MU)- obesity were 1.74 (1.67-1.81) and 1.76 (1.69-1.83), respectively. The transition from MH-overweight/obesity to MU-overweight/obesity increased the risk of AF (1.35, 0.97-1.88). CONCLUSION: BMI decrease of over 2% per year was associated with a lower risk of AF, particularly in those changing from obesity to overweight. The transition from MH-overweight/obesity to MU-overweight/obesity increased the AF risk. Weight management and maintenance of metabolic health should be recommended for primary prevention of AF.