The period of the symbiotic binary star AG Peg has been determined from photoelectric data, including previously unpublished data from the 1970s, as well as new observations. Altogether, 89 V magnitudes are available for discussion. These are corrected for a discrepancy in the magnitude of a comparison star used by various workers, and for a secular decline in the system's visual brightness of 0.021 mag/yr. Five different methods of period determination yield consistent results with a mean of 816.5 plus or minus 0.9 days, leading to an ephemeris for times of maximum visual light of (JD)max = 2442710.1 + 816.5E. A light curve based on normal points formed from the 89 individual observations is shown.