Despite four decades of research, the origin of Valles Marineris on Mars, the longest trough system in the solar system, remains uncertain. Its formation mechanism has been variably related to rifting, strike-slip faulting, and subsurface mass removal. This study focuses on the structural geology of Ius and Coprates Chasmata in southern Valles Marineris using THEMIS (Thermal Emission Imaging System), Context Camera (CTX), and HiRISE (High Resolution Imaging Science Experiment) images. The main result of the work is that the troughs and their plateau margins have experienced left-slip transtensional deformation. Syntectonic soft-sediment deformation suggests the presence of surface water during the Late Amazonian left-slip tectonics in Valles Marineris. The total left-slip motion of the southern Valles Marineris fault zone is estimated to be 150–160 km, which may have been absorbed by east-west extension across Noctis Labyrinthus and Syria Planum in the west and across Capri and Eos Chasmata in the east. The discovery of a large-scale (>2000 km in length and >100 km in slip) and rather narrow (<50 km in width) strike-slip fault zone by this study begs the question of why such a structure, typically associated with plate tectonics on Earth, has developed on Mars.