| Honorific Prefix: | Saint |
| Nissen | |
| Birth Date: | 5th century |
| Birth Place: | Ireland |
| Residence: | Mountgarret, County Wexford, Ireland |
| Death Date: | 5th century |
| Feast Day: | 25 July |
Saint Nissen was an early Christian convert who was abbot of a monastery in County Wexford, Ireland.
Saint Nissen was converted to Christianity by Saint Patrick. He became abbot of Montgarth (Mountgarret) Abbey in Wexford, Ireland.His feast day is 25 July.
The monks of St Augustine's Abbey, Ramsgate, wrote in their Book of Saints (1921),
The hagiographer Alban Butler (1710–1773) wrote in his Lives of the Primitive Fathers, Martyrs, and Other Principal Saints, under April 9,