We previously reported that induction of the adipocyte-specific gene adiponectin (Adipoq) during 3T3-L1 adipocyte differentiation is closely associated with epigenetic memory histone H3 acetylation on the transcribed region of the gene. We used 3T3-L1 adipocytes and Brd4 heterozygous mice to investigate whether the induction of Adipoq during adipocyte differentiation is regulated by histone acetylation and the binding protein bromodomain containing 4 (BRD4) on the transcribed region. Depletion of BRD4 by shRNA and inhibition by (+)-JQ1, an inhibitor of BET family proteins including BRD4, reduced Adipoq expression and lipid droplet accumulation in 3T3-L1 adipocytes. Additionally, the depletion and inhibition of BRD4 reduced the expression of many insulin sensitivity-related genes, including genes related to lipid droplet accumulation in adipocytes. BRD4 depletion reduced P-TEFb recruitment and histone acetylation on the transcribed region of the Adipoq gene. The expression levels of Adipoq and fatty acid synthesis-related genes and the circulating ADIPOQ protein level were lower in Brd4 heterozygous mice than in wild-type mice at 21 days after birth. These findings indicate that BRD4 regulates the Adipoq gene by recruiting P-TEFb onto acetylated histones in the transcribed region of the gene and regulates adipocyte differentiation by regulating the expression of genes related to insulin sensitivity.