The Polochic-Motagua strike-slip fault system in Guatemala marks the on-land plate boundary between the North American (NA) and the Caribbean (CA) plates. GPS observations in 1999 and 2003 show that the far-field velocity across the system (NA-CA relative velocity) is ~20 mm/yr. This is significantly higher than the NUVEL-1A velocity but is consistent with the GPS based CA-NA velocity proposed by DeMets et al. (2000). The observations are modeled by a fault centered on the Motagua fault, locked at a depth of 20 km, with a slip-rate decreasing from eastern to central Guatemala from 20 to 12 mm/yr towards the NA-CA-Cocos triple junction. This decrease is accommodated by ~8 mm/yr of E-W extension in the westernmost part of CA south of the Motagua fault. About 10 mm/yr of dextral slip is observed across the Mid-American Volcanic Arc. The NA-CA-Cocos triple junction is thus a complex, ~400 km-wide wedge-shaped area.