Fucoxanthin is converted to fucoxanthinol and amarouciaxanthin A in mice. It was previously reported that fucoxanthinol attenuated the adipogenesis of 3T3-L1 cells. However, the effects of amarouciaxanthin A on adipocyte differentiation have not been clarified. This study examined the effects of amarouciaxanthin A on 3T3-L1 adipogenesis by comparing the effects of fucoxanthinol, isofucoxanthinol, and amarouciaxanthin B. Amarouciaxanthin A significantly decreased glycerol-3-phosphate dehydrogenase (GPDH) activity, which was measured as an indicator of adipocyte differentiation. The suppressive effect of amarouciaxanthin A was stronger than that of fucoxanthinol, amarouciaxanthin B, and isofucoxanthinol. The mRNA expressions of adipocyte fatty acid-binding protein (aP2), lipoprotein lipase (LPL), and glucose-transporter 4 (Glut4) in 3T3-L1 cells were markedly down-regulated by amarouciaxanthin A compared to fucoxanthinol. Furthermore, the expression levels of peroxisome proliferator-activated receptor γ (PPARγ) and CCAAT-enhancer-binding protein α (C/EBPα),, which are the key adipogenic transcriptional factors, were also decreased by amarouciaxanthin A during adipocyte differentiation. These results show that amarouciaxanthin A, which is a dominant metabolite of fucoxanthin in white adipose tissue, suppressed 3T3-L1 adipocyte differentiation.