Generating new bispecific antibodies typically requires extensive engineering and cloning. New technologies such as made-to-order genes, advanced expression systems, and high efficiency cloning have simplified and accelerated this process. However, the timeline to create new bispecific antibodies is in the order of months, the resulting yield can be low, and adequate functionality is not guaranteed.Here, we developed a simple method to produce bispecific antibodies by site-specifically and covalently attaching a T cell redirecting domain to any off-the-shelf, human Immunoglobulin G (IgG). Our method excludes antibody engineering, cloning or prior knowledge of the antibody sequence, and the bispecifics are created in two hours. We demonstrated the applicability by assessing in vitro cytolysis of bispecific antibodies made using three FDA-approved monoclonal antibodies. We also show the ability to create personalized T cell redirecting autoantibodies (TRAAbs) by labeling native antibodies isolated from tumor-bearing mice, including two syngeneic murine models. TRAAbs were found to induce anti-tumor effects in vitro and in vivo. TRAAbs were also found to preferentially bind tumor over healthy tissue, highlighting a window for safe therapy. The use of autoantibodies to direct the tumor targeting of bispecific antibodies represents a new paradigm in personalized medicine that eliminates the need to identify tumor biomarkers and can potentially overcome tumor antigen loss, which commonly results from mono-targeted immunotherapies.