We present a catalog of ~390000 stars with proper motions {mu}>=20mas/yr and magnitudes r'<=20, drawn from the intersection of the SDSS Data Release One (see http://www.sdss.org/dr1) and USNO-B (<I/284>). We first use SDSS quasars to correct the proper motions in each of the two source catalogs, finding that these corrections for systematics are comparable to the statistical errors. The combined proper motions have errors of {sigma}{mu}=3.9mas/yr, significantly better than any previous catalog at these magnitudes. While cross-correlating SDSS and USNO-B removes the vast majority of the very large number of spurious proper-motion stars in each, some contamination remains, especially at faint magnitudes, r'>~19.5, and high proper motions. We present a diagrammatic method to estimate the level of contamination in selected subsets of the catalog, which should be useful for choosing appropriate selection criteria for various applications.