Current deformation of SE-Asia has been discussed controversially. The eastern margin of Eurasia, from North China to Indonesia, has been successively considered as rigidly attached to Eurasia¹, continuously deforming², or formed by a mosaic of blocks deforming mainly at their boundaries^3-6. Modern space geodetic tools can resolve these controversies by quantifying crustal deformation. Here we show results acquired using GPS data taken over a large part of SE Asia, that indicate that Sundaland, i.e. Indochina along with the western and central part of Indonesia, constitutes a stable tectonic block moving approximately east with respect to Eurasia at a velocity of 12±3 mm yr¹. With respect to India and Australia this block moves due south. Significant motion has not been detected along the northern boundary to S. China i.e. along the Red River Fault, whereas nearly 50 mm yr ¹ of right lateral motion has to be accommodated between India and Sundaland in the Andaman-Myanmar region.