The island of Puerto Rico is located within the complex boundary between the North America and Caribbean plates. The relative motion along this boundary is dominantly left-lateral strike slip, but compression and extension are locally significant. Although tectonic models proposed for the region suggest the presence of onshore active faults in Puerto Rico, no faults with Holocene displacement have been documented on the island before this study. Current seismic hazard assessments primarily consider only the impact of distant, offshore seismic sources because onshore fault hazard is unknown. Our mapping and trenching studies demonstrate Holocene surface rupture on a previously undocumented fault in southwestern Puerto Rico. We excavated a trench across a scarp near the southern edge of the Lajas Valley that exposed a narrow fault zone disrupting alluvial deposits. Structural relations indicate valley-side-down fault slip, with a component of strike-slip motion. Radiocarbon analyses of organic material collected from the sediments suggest that the most recent surface rupture occurred during the past 5000 yr, but no minimum age has yet been established. This fault may be part of a larger fault zone that extends from the western end of the Lajas Valley toward Ponce, the second largest city in Puerto Rico. ?? 2005 Geological Society of America.