| Otto I | |
| Landgrave of Hesse | |
| Spouse: | Adelheid of Ravensberg |
| Issue: | Henry II the Iron Otto, Archbishop of Magdeburg Louis the Junker Hermann I the Elder Elisabeth |
| Noble Family: | House of Hesse |
| Father: | Henry I, Landgrave of Hesse |
| Mother: | Adelheid of Brunswick-Lüneburg |
| Birth Place: | Marburg |
| Death Date: | 17 January 1328 |
| Death Place: | Kassel |
Otto I (17 January 1328) was Landgrave of Upper Hesse from 1308 and then Landgrave of Lower Hesse from 1311 until his death.[1]
Otto was born in Marburg, a son of Henry I, Landgrave of Hesse and his first wife Adelheid of Brunswick-Lunenburg. Following the death of his father in 1308, he inherited Upper Hesse, the "Land on the Lahn", which included Marburg, Giessen, Grünberg and Alsfeld. After his half-brother John died in 1311, he also became landgrave of Lower Hesse. This included the area below the Fulda, Eder, Schwalm, Werra, and the upper reaches of the Weser, with the residence of Kassel, as well as the towns of Homberg (Efze), Melsungen, and Rotenburg an der Fulda.
After Otto inherited the whole Landgraviate, he resided alternately in Kassel and Marburg. Otto had a long conflict with the Archbishop of Mainz, Matthias von Bucheck. In 1324 Mainz won the battle in the Lahnberg with help from troops from Amöneburg. In 1327, Mainz adopted Giessen with the help of troops from the Electorate of Trier, but the citizens of the town quickly enabled Otto to regain control. Only after Otto's victory at Wetzlar and with the mediation of King John of Luxembourg in 1328 was conciliation made possible.
Otto died on 17 January 1328 in Kassel, and was buried in the Ahnaberg monastery.
He was married 1297 in Saint Elisabethkirche, Marburg to Adelheid of Ravensberg, daughter of Count Otto III of Ravensberg and had the following children: