To describe a novel pathogenic ADCY3 variant implicated in early-onset obesity and functionally characterize this variant via in vitro and in silico validation, we identified a novel homozygous nonsense variant c.2520C>G, p.Thr840X in the ADCY3 gene using gene panel sequencing in a four-year-old girl. This evidence concerns the gene ADCY3 and obesity due to melanocortin 4 receptor deficiency.