Abstract
The author has earlier defined the quantum two-time localization problem as the minimizing of a particle's position spread about specified points at two distinct times. In the present article optimum localization is found for relativistic massive free particles. For short time intervals, spreading necessarily occurs at the speed of light while for long times the previously found diffusion-like behaviour is recovered. In defining relativistic localization, use is made of the work of Newton and Wigner (1949); in particular, their restriction to the positive energy hyperboloid is found to be necessary to recover the nonrelativistic limit of wavepacket spreading.