Aripiprazole (ARP), a quinolinone derivative, is an atypical antipsychotic drug that is used in the treatment of schizophrenia. ARP has an extensive distribution and more than 99% of the ARP and dehydro-ARP, the main active metabolite, is bound to plasma proteins. However, information regarding the protein binding of ARP is limited. In this study, we report on a systematic study of the protein binding of ARP. The interaction of ARP and structurally related compounds with human serum albumin (HSA) was examined using equilibrium dialysis, circular dichroism (CD) spectroscopy, fluorescent probe displacement, and an X-ray crystallographic analysis. The binding affinities (nK) for ARP and its main metabolite, dehydro-ARP with HSA were found to be significantly higher than other structurally related compounds. The results of equilibrium dialysis experiments and CD spectral data indicated that the chloro-group linked to the phenylpiperazine ring in the ARP molecule plays a major role in the binding of these ligands to HSA. Furthermore, fluorescent probe displacement results indicated that ARP appears to bind at the site II pocket in subdomain III. A detailed CD spectral analysis suggests that the chloro-group linked to the phenylpiperazine ring may control the geometry of the ARP molecule when binding in the site II binding pocket. X-ray crystallographic analysis of the ARP-HSA complex revealed that the distance between the chlorine atom at the 3-positon of dichlorophenyl-piperazine on ARP and the sulfur atom of Cys392 in HSA was 3.4-3.6 Å. A similar halogen bond interaction has also been observed in the HSA structure complexed with diazepam, which also contains a chloro-group. Thus, the mechanism responsible for the binding of ARP to a protein elucidated here should be relevant for assessing the pharmacokinetics and pharmacodynamics of ARP in various clinical situations and for designing new drugs.