| Stefan Niesiołowski | |
| Nationality: | Polish |
| Office: | Marshal of the Sejm |
| Term Start: | 8 July 2010 |
| Term End: | 8 July 2010 |
| Predecessor: | Bronisław Komorowski |
| Successor: | Grzegorz Schetyna |
| Term Start2: | 5 November 2007 |
| Term Start3: | 19 October 2005 |
| Term End3: | 4 November 2007 |
| Party: | Solidarity (before 1991) Christian National Union (1991-2001) Right Alliance (2001-2003) Civic Platform (2005-2016) Union of European Democrats (2016-present) |
| Birth Date: | 1944 2, df=y |
| Birth Place: | Kałęczew, Poland |
| Signature: | Stefan Niesiołowski signature.jpg |
| Children: | daughter |
| Spouse: | Anna Królikowska-Niesiołowska |
Stefan Konstanty Myszkiewicz-Niesiołowski (pronounced as /pol/; born 4 February 1944) is a Polish politician and member of the Union of European Democrats.In 1970, Niesiołowski became involved in anti-communist opposition Ruch. He was the main proponent of burning down the Lenin Museum in Poronin, for which he was sentenced to 7 years imprisonment in 1971, and was released under amnesty in 1974. In 1980, Niesiołowski joined the Independent Self-governing Trade Union "Solidarity". During the martial law, Niesiołowski was imprisoned in Jaworze for around a year.