Detailed structural analysis was conducted on the E-W trending Tenango Fault System (TFS) at the western end of the La Pera Fault System, a major structure within the Trans-Mexican Volcanic Belt (TMVB). Field mapping, which included trenching the fault trace, revealed the displacement of recent volcanic deposits. The study indicates that the TFS is an active subvertical fault with transtensive left-lateral movement consistent with the contemporary stress regime in the region. The estimated Holocene slip rate is 0.3--0.5 mm/yr. The TFS activity constitutes an important seismic hazard in the intra-arc Lerma basin. Several towns are located within this basin and are particularly vulnerable to seismic risk because of its underlying strata consisting of lacustrine and volcaniclastic deposits (estimated maximum Mw 6.8--7). The TFS is an important structural element in the complex tectonic deformation history of central eastern TMVB and influenced the behavior of recent volcanism within the Lerma basin.