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.

In 894 and again in the winter of 895/6, Hatto accompanied the Emperor on his campaigns in Italy. After the death of Arnulf in 899, Hatto was fundamental in electing his infant son Louis King, and was active in his regency. In 906 in the Babenberger Feud Hatto aligned with the Conradines to defeat and kill the rebellious Adalbert at Fritzlar after breaking his promise to escort him safely, thus allowing them to take over Franconia. The friendship between Hatto and the Conradines was confirmed after the death of King Louis IV in the election of Conrad I to the kingship of Germany. Hatto later attempted to kill Duke Henry the Fowler of Saxony secretly. Hatto died in 913.

Preceded by:

Hatto I

Succeeded by:

Sunderold Archbishop of Mainz
891 - 913
Ruodho Abbot of Reichenau
888 - 913
Salomo Abbot of Ellwangen
896 - 913
Adalbero of Dillingen