Global Positioning System (GPS) measurements provide the first direct measurement of plate motion and crustal deformation across the Scotia-South America transform plate boundary in Tierra del Fuego. This plate boundary accommodates a part of the overall motion between South America and Antarctica. The subaerial section of the plate boundary in Tierra del Fuego, about 160 km in length, is modeled as a two dimensional, strike-slip plate boundary with east-west strike. Along the Magallanes-Fagnano fault system, the principal fault of this portion of the plate boundary, relative plate motion is left-lateral strike-slip on a vertical fault at 6.6 ¿ 1.3 mm/year based on an assumed locking depth of 15 km. The site velocities on the Scotia Plate side are faster than the relative velocity by an additional 1--2 mm/yr, suggesting there may be a wider region of diffuse left-lateral deformation in southern Patagonia. The north-south components of the velocities, however, do not support the existence of active, large-scale transpression or transtension between the South America and Scotia plates along this section of the plate boundary.