Hydrochloric thiothixene (HT) is an antipsychotic drug used in the treatment of various psychoses including schizophrenia, mania, polar disorder, and in behavior disturbances. However, because the psychotics often could not control their behaviors, the independent administration of antipsychotic drug based on medical order was difficult. The omissions of the administration often brought an unsatisfactory therapeutic efficacy. A novel injectable long-term control-released in situ gel of HT for the treatment of schizophrenia was developed based on biodegradable material polylactic acid (PLA). The optimum formulation of the injectable PLA-based HT in situ gel containing 15% (w/w) HT and 45% (w/w) PLA with benzyl benzoate was used as a gelling solvent. The results of the in vitro and in vivo studies showed that this in situ gel had a long-term period of drug release for several weeks and a good histocompatibility without any remarkable inflammatory reactions.