The natural evolution of new catalytic functions is attributed to the existence of latent (promiscuous) activities in enzymes. Herein, we present the first example of catalytic promiscuity of alkaline phosphatase from the primary active site that can catalyze the hydrolysis of aryl α-glucosides, in addition to their well-explored substrate promiscuity. To explore structural features, various aryl glycosides were synthesized, which showed the hydrolysis of only a specific class of α-glucosides with a free neighboring hydroxyl group on the aryl moiety. Thus, our discovery has implications for the evolution of enzyme functions and hidden diversity.