The San Andres rift (SAR), located on the lower Nicaraguan Rise, is a previously poorly studied, active, 015 degrees-trending, bathymetric, and structural rift basin that is 11-27 km (7-17 mi) wide and extends for 346 km (215 mi) across the western flank of the Caribbean plate. In this study, we integrate bathymetric maps, potential field data, and high-resolution, two-dimensional (2-D) seismic lines to understand the crustal structure, tectonic history, and tectonic origin of the SAR, which is one of the active areas within the otherwise stable Caribbean plate. We compiled regional gravity and magnetic data that revealed a negative gravity anomaly and positive magnetic anomaly that we interpret as a result of crustal thinning and an elevated Moho along the main rift axis of the SAR. Forward models of gravity data show four possible interpretations for the origin of the crust underlying and surrounding the SAR. Interpretations of 2-D seismic reflection data show structural features within the upper crust and sedimentary sections typical of other active rift systems including a SAR-parallel, north-south alignment of earthquakes with the larger events showing normal and strike-slip focal mechanisms. Sequential kinematic restorations based on 2-D seismic profiles reveal three major phases of SAR opening: (1) the initial early Eocene rifting stage; (2) middle Eocene extension; and (3) a rapid middle Miocene to early Pliocene extension accompanied by emergence of the San Andres Island as a rift shoulder. We propose slab rollback and intraplate extension as main tectonic mechanisms to explain all rift phases and Neogene volcanism found in the western Caribbean region.