Using gridded MBT and XBT data for the period 1955--2002, we analyze the interdecadal variability of the thermocline along the western coast of South America between the equator and 32¿S. Results show a relatively small, but coherent, interdecadal oscillation of the depth of different isotherms representing the thermocline along the region. This oscillation is well correlated with interdecadal SST anomalies. At the end of the 1960s the thermocline was on average 10 m shallower than during the beginning of the 1980s, while SST anomalies changed from -0.3¿C to 0.5¿C during the same period. The transition from shallow to deep thermocline depths during the mid-1970s is consistent with the change from cold to warm conditions observed at the surface. Changes in the depth of the thermocline base may modify the properties of the subsurface water that feeds the coastal upwelling, with further consequences for the ecosystem of the region.