Accurate reconstruction of sea surface temperature (SST) is a high research priority, given that it is such a crucial variable in the Earth's climate system. The Mg/Ca composition of Globigerinoides ruber (white) has been calibrated and applied for a number of tropical and extratropical paleo-SST reconstructions, though validation studies of the proxy against instrumental observations are relatively scarce. Here we present a validation of G. ruber Mg/Ca-derived SSTs against instrumental summer values, firstly from the modern seasonal water column perspective, and secondly from a 20th century observational time series. The study occurs in the San Lzaro Basin (SLB), one of the marginal basins in the NE Pacific known for very high sedimentation rates, excellent preservation, laminated sequences, and the ability to record upwelling processes on high-resolution timescales, from interannual climatic variability (El Nio/Southern Oscillation (ENSO)) to interdecadal (e.g., the Pacific Decadal Oscillation (PDO)). Results suggest that the proxy best reflects the summer season. The proxy-instrument time-series comparison for summer SSTs displays remarkable agreement, driven largely by ENSO cycles for the past century, with some events missing due to scarcity of foraminiferal specimens and/or lack of sufficient temporal resolution. This study validates the G. ruber Mg/Ca proxy for summer SSTs in this region, and suggests its high fidelity to reconstruct summer SST from SLB over longer timescales to record multi-decadal and multi-centennial variabilities. Copyright 2011 by the American Geophysical Union.