Graft‐versus‐host disease (GVHD) caused by donor T cells attacking recipient tissues is a major cause of morbidity and mortality following allogeneic hematopoietic stem cell transplantation (alloSCT). Studies have shown that effector memory T (T_EM) cells do not cause GVHD but are capable of immune functions post‐transplant, including graft‐versus‐leukemia (GVL) effects, but the reasons for this are unclear. In mice, the T_EM pool may have a less diverse T‐cell receptor (TCR) repertoire than naive T (T_N) cells with fewer alloreactive clones. We therefore tested whether enhancing the alloreactivity of T_EM cells would restore their ability to cause GVHD. In an MHC‐matched system, alloreactive T_EM cells were created by transferring GVHD effector cells into syngeneic recipients and allowing conversion to T_EM cells. Upon retransfer to freshly transplanted recipients, these cells caused only mild GVHD. Similarly, in an MHC‐mismatched system, T_EM cells with a proven increased precursor frequency of alloreactive clones only caused limited GVHD. Nonetheless, these same cells mounted strong in vitro alloresponses and caused rapid skin graft rejection. T_EM cells created from CD4⁺ T cells that had undergone lymphopenia‐induced proliferation (LIP) also caused only mild GVHD. Our findings establish that conversion to T_EM cells significantly reduces GVHD potency, even in cells with a substantially enhanced alloreactive repertoire.