Exosomes are naturally occurring membranous vesicles secreted by various types of cells. Given their unique and important biological and pharmacological properties, exosomes have been emerging as a promising form of nanomedicine acting via efficient delivery of endogenous and exogenous therapeutics. Here we explore a new concept of utilizing endogenously derived exosomes as artificial controllers of cellular immunity to redirect and activate cytotoxic T cells toward cancer cells for killing. This was achieved through genetically displaying two distinct types of antibodies on exosomal surface. The resulting synthetic multivalent antibodies retargeted exosomes (SMART-Exos), which express monoclonal antibodies specific for T-cell CD3 and cancer cell-associated epidermal growth factor receptor (EGFR), were shown to not only induce cross-linking of T cells and EGFR-expressing breast cancer cells but also elicit potent antitumor immunity both in vitro and in vivo. This proof-of-concept study demonstrates a novel application of exosomes in cancer immunotherapy and may provide a general and versatile approach for the development of a new class of cell-free therapy.