| Ragna Nielsen | |
| Office1: | President of the Norwegian Association for Women's Rights |
| Term Start1: | 1886 |
| Term End1: | 1888 |
| Predecessor1: | Anna Stang |
| Successor1: | Anna Bugge |
| Term Start2: | 1889 |
| Term End2: | 1895 |
| Predecessor2: | Anna Bugge |
| Successor2: | Randi Blehr |
| Office3: | 2nd President of the Riksmål Society |
| Term Start3: | 1910 |
| Term End3: | 1911 |
| Predecessor3: | Bjørnstjerne Bjørnson |
| Successor3: | Alfred Eriksen |
| Birth Date: | 17 July 1845 |
| Death Date: | 29 September 1924 |
| Nationality: | Norwegian |
| Mother: | Vilhelmine Ullmann |
| Relatives: | Viggo Ullmann (brother) |
| Birth Name: | Ragna Vilhelmine Ullmann |
Ragna Vilhelmine Nielsen (née Ullmann) (17 July 1845 – 29 September 1924) was a Norwegian pedagogue, school headmistress, publicist, organizer, politician and feminist.
Ragna Nielsen was born in Christiania (now Oslo) to Jørgen Axel Nicolai Ullmann and his wife, pedagogist, publicist, literary critic and feminist Cathrine Johanne Fredrikke Vilhelmine Dunker. She married Ludvig Nielsen in 1879, and settled with her husband in Tromsø. The couple was separated in 1884, when she moved back to Kristiania. She was the sister of politician Viggo Ullmann.
As a child, Ragna attended her mother's school for girls, and then attended Hartvig Nissen's private school for girls until 1860. From 1862, she received an assignment at Nissen's school, where she taught until 1879. She was a teacher in Tromsø until 1884. She established the school in Kristiania in 1885. It was started as a girls' school, but soon became a common school for both girls and boys. She was the first headmistress of a secondary school (Norwegian: [[Gymnasium (school)#Nordic and Baltic gymnasiums|gymnas]]) in Norway. She chaired the Norwegian Association for Women's Rights twice, from 1886 to 1888, and again from 1889 to 1895. She founded or co-founded a number of organizations, including in 1885, in 1890, in 1891, and Hjemmenes Vel in 1898. She was elected to the Kristiania City Council from 1901 to 1904. She co-founded the language organization Riksmålsforeningen in 1907, and chaired the organization from 1909 to 1910. She was engaged in the spiritualism movement and a co-founder of in 1917.
She co-founded the women's magazine in 1921. Among her books are from 1904, from 1907 (published anonymously), and from 1922.