The Zwicky Catalog of Galaxies, with a magnitude limit m<sub>Zw</sub> <= 15.5, has been the basis for the Center for Astrophysics (CfA) redshift surveys. To date, analyses of the Zwicky Catalog and redshift surveys based on it have relied on heterogeneous sets of galaxy coordinates and redshifts. In this Updated Zwicky Catalog (UZC), some of the inadequacies of previous catalogs are corrected by providing (1) coordinates with ~<2 arcsecond errors for all of the 19,369 catalog galaxies, (2) homogeneously estimated redshifts cz (radial velocities) for the majority (98%) of the data taken at the CfA (14,632 spectra), and (3) an estimate of the remaining "blunder" rate for both the CfA redshifts and for those compiled from the literature. For the reanalyzed CfA data a calibrated, uniformly determined error and an indication of the presence of emission lines in each spectrum are included. Redshifts (radial velocities) are provided for the 7257 galaxies in the CfA2 redshift survey that were not previously published; for another 5625 CfA redshifts (radial velocities), the remeasured or uniformly rereduced values are listed. Among the new measurements, 1807 are members of UZC "multiplets" associated with the original Zwicky catalog position in the coordinate range where the catalog is 98% complete. These multiplets provide new candidates for examination of tidal interaction among galaxies. All of the new redshifts (radial velocities) correspond to UZC galaxies with properties recorded in the CfA redshift compilation known as ZCAT. The redshift catalog included in the UZC is ~96% complete to m<sub>Zw</sub> <= 15.5 and ~98% complete (12,925 galaxies out of a total of 13,150) for the right ascension ranges 20 hr >= RA(1950) <= 4 hr and 8 hr <= RA(1950) <= 17 hr and the declination range -2.5 degrees <= Dec(1950) <= 50 degrees. This more complete region includes all of the CfA2 survey as analyzed to the date of the publication of the UZC (1999). This database was created by the HEASARC in October 2000 based on a machine-readable version obtained from the CDS (Catalog J/PASP/111/438). It was slightly revised in February 2001 (the 'redshift' parameters were renamed as 'radial velocity' parameters to conform with the usage in other similar HEASARC extragalactic catalogs). This is a service provided by NASA HEASARC .