A major mechanism by which human regulatory T cells (T_reg) have been shown to suppress and kill autologous immune cells is through the granzyme-perforin pathway. However, it is unknown whether T_regs also possess the capacity to kill tumor cells using similar mechanisms. Bispecific antibodies (bscAb) have emerged as a promising class of therapeutics that activate T cells against tumor antigens without the need for classical MHC-restricted T-cell receptor (TCR) recognition. Here, we show that a bscAb targeting the tumor-specific mutation of the EGF receptor, EGFRvIII, redirects human CD4⁺CD25⁺FoxP3⁺ T_regs to kill glioblastoma cells. This activity was significantly abrogated by inhibitors of the granzyme-perforin pathway. Notably, analyses of human primary glioblastoma also displayed diffused infiltration of granzyme-expressing FoxP3⁺ T cells. Together, these data suggest that despite their known suppressive functions, tumor-infiltrating T_regs possess potent cytotoxic mechanisms that can be co-opted for efficient tumor cell lysis. Cancer Immunol Res; 1(3); 163–7. ©2013 AACR.