| Birth Date: | 16 November 1857 |
| Birth Place: | Möðruvellir in Hörgárdalur, Iceland |
| Death Place: | Cologne, Germany |
| Occupation: | Children's writer |
Jón Stefán Sveinsson, SJ, better known as Nonni (16 November 1857 - 16 October 1944), was an Icelandic children's writer and member of the Society of Jesus.
He left Iceland in 1870 for France, where he converted to Catholicism.
His children's stories concerning a character named Nonni are well known in Iceland and parts of Europe. Nonni's House (Nonnahús), his childhood home in Akureyri, is now a museum dedicated to his life and work, and he has been featured on postage stamps. Jón Sveinsson's stories of growing up with his brother Ármann, nicknamed "Manni", were adapted into an Icelanic-West German television series Nonni and Manni.
Titles are currently listed in German.