| Otto III, Count of Rietberg | |
| Noble Family: | House of Rietberg |
| Father: | John I, Count of Rietberg |
| Mother: | Margaret of Lippe |
| Spouse: | Anna of Sayn Onna of Esens |
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| Birth Date: | between 1475 and 1485 |
| Burial Place: | Marienfeld Abbey |
Count Otto III Rietberg (between 1475 and 1485 - 18 December 1535) was Count of Rietberg from 1516 to 1535.
Otto III was probably born between 1475 and 1485 as the eldest son of John I and his wife Margaret of Lippe.
Otto's second wife, Onna of Esens, introduced the Reformation in the County of Rietberg. In 1533, Otto III appointed two preachers from Lippe, to teach the new faith in the parishes of Rietberg and Neuenkirchen.
Otto III suffered from ulcers on his leg. He died on 18 December 1535 and was buried in the Marienfeld Abbey. He was the last Count of Rietberg to be buried there. On his gravestone in the cloister before the winter refectory a Latin text could be read:
In the year of our Lord 1535, on 18 December, died
Noble Lord Otto, Count and Lord of Rietberg.
May his soul rest in peace.
Beside it was the grave stone of his first wife:
In the year of our Lord 1523, the day before St. Anthony, died
Anna of Sayn, Countess in Rietberg.
May her soul may in peace.
Otto III's first marriage, which was closed before 1515, was with Countess Anna of Sayn (died: 16 January 1523). With her he had five children:
After Anna's death, Otto III married on 26 September 1526 Onna of Esens (died after 1559). She was the daughter of Hero Oomkens von Esens, an East Frisian chieftain, and the sister of Balthasar Oomkens von Esens of the House of Attena. Via this marriage, the Counts of Rietberg acquired the Harlingerland. In 1526, it was held by Onna's brother Balthasar. Five years after Otto's death, Balthasar died childless and Onna inherited his land.
Otto had one child with Onna:
Otto III had an illegitimate daughter with Jutte, who later married John of Willen, the Seneschal of the County of Rietberg: