The Atirro–Río Sucio fault system forms a major northwest-trending strike-slip fault zone in east-central Costa Rica. We examined the kinematics and temporal evolution of this fault system through geomorphic, structural, and seismologic analysis. This 150-km-long strike-slip fault zone traverses the northern flank of the paleovolcanic Cordillera de Talamanca and extends northwestward into the active Cordillera Volcánica Central. Historical seismicity includes frequent minor swarms and occasional moderate-magnitude (M 5.0–6.5) damaging earthquakes. Field geomorphic evidence, fault kinematic data, and earthquake focal mechanisms are consistent in showing dextral slip along the mapped traces of northwest-striking faults. Continuity with other transcurrent faults in northwest Costa Rica indicates that the Atirro–Río Sucio fault system may form the southeastern end of a regional network of northwest-trending dextral faults that accommodate margin-parallel displacement of the Central American forearc sliver. The Atirro–Río Sucio fault system originates within the Central Costa Rica Deformed Belt inboard of the indenting Cocos Ridge. We infer that ridge collision drives lateral escape of crustal fragments northwestward along an array of dextral Central Costa Rica Deformed Belt faults including the major structures of the Atirro–Río Sucio fault system. This zone of arc-parallel extrusion thus represents the root of the Central American forearc sliver. Consistent with recent geodynamic models, we propose that northwestward sliver escape along the Atirro–Río Sucio faults is driven by rigid indentation of the aseismic Cocos Ridge into southern Costa Rica.