Genome-wide association studies have revealed that single nucleotide polymorphisms (SNPs) in the first intron of the fat mass and obesity associated (FTO) gene are associated with obesity in humans. Animal studies have since demonstrated that mice overexpressing FTO (FTO-4) have increased body weight compared to wild type (WT) mice. In spite of this, the mechanisms involved in FTO-induced adiposity remain elusive, particularly at the level of the adipocyte. We observed that the adipogenic genes CEBPα and PPARγ were upregulated (2.85±0.17- and 2.14±0.16-fold, respectively) in gonadal adipose tissue of FTO-4 mice compared to WT. We also observed that primary preadipocytes derived from FTO-4 mice exhibit increased triglyceride accumulation and lipid droplet size following adipogenic stimulation compared to that of WT mice. Gene expression analysis of these cells confirmed that differentiated preadipocytes from FTO-4 mice had a significantly higher expression of adipogenic genes, including PPARγ (56.80±8.07-fold), FABP4 (51.26±9.71-fold) and PLIN1 (74.17±6.44-fold) after 10 days of treatment with an adipogenic cocktail compared to those of WT mice. Similar findings were observed in MEFs derived from FTO-4 mice, in which adipogenic gene expression was significantly elevated compared to WT MEFs after 3 days of differentiation. Conversely, MEFs derived from FTO knockout mice exhibited lower adipogenic gene expression compared to WT MEFs following adipogenic stimulation. Given that FTO-dependent changes in adipogenesis were observed within 3 days of adipogenic stimulation, we hypothesized that FTO was inducing adipogenesis by influencing the early proliferation phase of adipogenesis known as mitotic clonal expansion (MCE). MCE, a prerequisite for adipogenesis, is mainly regulated by Cyclin D1 and D3. We observed that in FTO-4 MEFs, transfection with FTO siRNA lead to a significant downregulation of Cyclin D1 at 24 hours (FTO-4 siCON 1.36±0.12 vs FTO-4 siFTO 0.76±0.04 relative to GAPDH) and Cyclin D3 at 40 hours (FTO-4 siCON 1.35±0.09 vs FTO-4 siFTO 0.62±0.03 relative to GAPDH) post adipogenic induction compared to FTO-4 MEFs transfected with control vector. In conclusion, our results suggest that FTO promotes adipogenesis through regulating MCE.