Background: Although obesity as a risk factor for various cardiovascular outcomes has been studied for decades, the results from numerous studies are heterogeneous and sometimes even contradictory to one another, making it difficult for clinicians and policy makers to determine which associations are genuine and reliable. Methods: We conducted an umbrella review of systematic reviews and re-analyses of meta-analyses to evaluate the strength and certainty of the evidence underlying an association between increased adiposity, as assessed by body-mass index (BMI), waist circumference (WC), or waist-to-hip ratio (WHR) and the risk of incident cardiovascular disease (CVD) events or mortality. Google Scholar, PubMed, Embase, and Cochrane Database of Systematic Reviews were screened, and a total of 11 systematic reviews and 53 meta-analyses that investigated associations between obesity and cardiovascular outcomes were included. Results from recently published cohort studies were also incorporated into the existing meta-analyses to update them with more recent data. Thus, the present study compiled all the relevant evidence accumulated to date, encompassing a total of 488 cohorts and over 30 million participants. Mendelian randomisation (MR) studies were collected to identify any causal relationship between obesity and various CVD outcomes, and to avoid reverse causality. The evidence levels of pooled results were graded into high, moderate, low, and very low according to the Grading of Recommendations Assessment, Development and Evaluation (GRADE) framework. Findings: An increase in BMI was associated with a higher risk of developing coronary heart disease, heart failure, atrial fibrillation, stroke, hypertension, aortic valve stenosis, pulmonary embolism, and venous thromboembolism; the study results corroborate the casual effect of obesity on the incidence of CVD, except stroke, based on MR studies. The increase in the risk of developing CVD for every 5 kg/m2 increase in BMI ranged from 7% (relative risk [RR], 1∙07; 95% confidence interval [CI], 1∙03 to 1∙11) for stroke to 49% (RR, 1∙49; 95% CI, 1∙41 to 1∙58) for hypertension. The risk of all-cause mortality and CVD-specific mortality increased with adiposity, which was supported by a high grade of evidence from observational analyses; however, the causal effect of obesity on mortality outcomes was not significant in MR studies. Interpretations: Only 15 out of the 53 associations (28%) reported for obesity and CVD outcomes were supported with high evidence levels from observational analyses. Although other reported associations might be valid, various degrees of uncertainty remain. The causal effect of obesity on 9 of the 14 CVD-related outcomes was corroborated by MR studies. As obesity is progressively increasing around the globe and because CVD remains a constant threat to public health, it is necessary to understand the gradient of evidence underlying the association between these two clinical entities. Any weak links in the association and causality discovered in this review should be reinforced with further scientific research, while high-certainty associations with established causality should be reflected in clinical practices and health policies. Systematic Review Registration: PROSPERO CRD42020179469. Funding Statement: This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (No. 2019R1A2C4070496). Declaration of Interests: All authors have completed the ICMJE uniform disclosure form at www.icmje.org/coi_disclosure.pdf (available on request from the corresponding author) and declare the following: no support from any organization for the submitted work; no financial relationships with any organizations that might have an interest in the submitted work in the previous 3 years; no other relationships or activities that could appear to have influenced the submitted work.