We report the detection of a transient gamma-ray source in the direction of B3 1428+422 ( z = 4.72) by analyzing the 110-month Fermi-LAT. Pass 8 data. The new transient gamma-ray source is far away from the Galactic plane and has a rather soft spectrum, in agreement with being a high redshift blazar. We suggest that the newly discovered transient is the gamma-ray counterpart of B3 1428+422, which could be the most distant GeV source detected so far. The detection of a group of such distant gamma-ray blazars will be helpful in reconstructing the evolution of the luminosity function and studying the extragalactic background light at such high redshifts.