Hatto I (c. 850 - 15 May 913) was the Archbishop of Mainz from 891 until 913, the Abbot of Reichenau from 888 until 913, and the Abbot of Ellwangen from 896 until 913. Hatto was born into a Swabian aristocratic family in circa 850. He was educated in the church, and in 887 was an advisor to King Arnulf of Carinthia. He was appointed the Abbot of Reichenau in Lake Constance in 888. Hatto was appointed by Arnulf the arch-chancellor of East Franconia, and in September 891 was appointed by him the Archbishop of Mainz, the most important powerful church position in medieval Germany. Hatto was appointed the abbot of Ellwangen in 896, although he allowed the monks of Lorsch, Weissenburg and Klingenmünster to elect their own abbots.