Eligiusz Niewiadomski (Warsaw, December 1, 1869 – January 31, 1923, Warsaw) was a Polish modernist painter and art critic who belonged to the right-wing National Democratic Party till 1904 and later continued supporting it. In 1922 he assassinated Poland's first President, Gabriel Narutowicz, in his first week in office as president.
Niewiadomski was born into a family of gentry descent. His father, Wincenty Niewiadomski, of the Prus coat-of-arms, was a veteran of the January Uprising and a worker at the Warsaw mint. At the age of two, Eligiusz lost his mother Julia, and was raised by his elder sister Cecylia. 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 graduated in 1894 with honors, and won a scholarship to the École Supérieure des Beaux-Arts in Paris. After his return to Warsaw, he became a student of Wojciech Gerson, one of the best-known Polish artists of the age.
After 1897, he taught drawing at the Warsaw Polytechnic. 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. Niewiadomski prepared and published a map of the Tatras, one of the first tourist maps of the area. He also prepared a set of historical maps of Poland, Album of the History of Poland (1899). He also became involved in the reorganization of the Zachęta art society. Using contacts acquired there, he promoted the idea of creating a separate Academy of Fine Arts in Warsaw. However, when the school was finally opened in 1903, Niewiadomski was not invited to teach there.