Prostate stem cell antigen (PSCA) is expressed in the majority of prostate cancer patients, making it an ideal target for cancer immunotherapy. Murine monoclonal antibody 1G8 binds to PSCA with nanomolar affinity, but its efficacy as a therapeutic agent is limited by the generation of a HAMA response. The present invention discloses humanized 1G8 antibodies in which the majority of the mouse-derived epitopes have been removed. These humanized antibodies bind PSCA with high affinity and specificity, and have been shown to reduce human bladder tumor take in a nude mouse model. These characteristics make the humanized antibodies of the present invention attractive agents for the treatment and detection of tumors expressing PSCA.