Eligiusz Józef Niewiadomski (1 December 1869 – 31 January 1923) was a Polish modernist painter and art critic who sympathized with the right-wing National Democracy movement.

After graduating from a local trade school in 1888, Niewiadomski moved to St. Petersburg, where he continued his studies at the Imperial Academy of Arts.

He also collaborated with a number of Warsaw-based magazines and newspapers as a journalist and art critic, which gave him considerable notoriety, mostly among the artists themselves.

He became involved in various artistic movements, among them the "re-discovery" of the Tatra Mountains, which at the time attracted some of the most renowned Polish painters, poets and writers as a source of inspiration.

To make a living, Niewiadomski began teaching art classes at numerous schools and churches in Poland.

After the outbreak of World War I he remained in Warsaw, where he published brochures and manifestos describing his views on the role of art.

On 1 March 1918 he was appointed director of painting and sculpture at the Regency Council's Ministry of Culture, a post that had previously been turned down by numerous artists.

During the last months of the war, he finally managed to convince his superiors to transfer him to front-line service and fought in the 5th Legions' Infantry Regiment.

However, on 8 November 1921, after Antoni Ponikowski's government refused to grant Niewiadomski's department a higher budget, he resigned his post.

A few months after his death, a renowned psychiatrist, Maurycy Urstein, called Niewiadomski's execution a judiciary homicide, as the perpetrator was mentally ill and clinically insane but the court had disregarded the need for expert psychiatric testimony.