In the 21st century, obesity and its related health complications have become a serious problem worldwide. The primary reasons for this situation are environmental and genetic factors. In this review, we focused on a genetic marker that was found to play a crucial role in determining an individual’s predisposition to weight gain and being obese. This dissertation will also have a look into genome-wide association studies (GWAS) that revealed the possible link of a single nucleotide polymorphism (SNPs) in the first intron of the Fat Mass Obesity Associated (FTO) gene to body mass index and obesity. Moreover, it has become clear that this association with the regulation of body weight is replicable across multiple populations. Furthermore, evidence from animal models where FTO expression has indicated the role of the FTO gene in the regulation of body weight. In the regulation of body weight, FTO protein disallows weight gain by increasing the body’s sensitivity to satiety to prevent excessive food intake through fat cell lipolysis. On the other hand, the FTO variant risk allele increased the appetite and energy intake, but not the expenditure of energy that can lead to weight gain. Individual that is having homozygous of the risk allele, which is the A-allele, was associated with the risk of being obese and the rise in BMI. The purpose of this review is to discuss the association of the FTO gene common variant, rs9939609 with obesity and body mass index, and to describe the role of the FTO gene in the regulation of body weight.