Provided herein are novel anti-CD28×anti-B7H3 (also referred to as “αCD28×αB7H3”) heterodimeric bispecific antibodies and methods of using such antibodies for the treatment of cancers. Subject αCD28×αB7H3 antibodies are capable of agonistically binding to CD28 costimulatory molecules on T cells and targeting to B7H3 on tumor cells. Thus, such antibodies selectively enhance anti-tumor activity at tumor sites while minimizing peripheral toxicity. The subject antibodies provided herein are particularly useful for enhancing anti-tumor activity when used in combination with other anti-cancer therapies.