| Mariusz Siembida | |
| Fullname: | Mariusz Siembida |
| Strokes: | backstroke |
| Club: | KS Lublinianka |
| Birth Date: | 21 March 1975 |
| Birth Place: | Puławy, Lubelskie |
Mariusz Siembida (born 21 March 1975 in Puławy, Lubelskie) is a retired backstroke swimmer from Poland, who competed for his native country at two consecutive Summer Olympics, starting in 1996 (Atlanta, Georgia).
A member of Lublinianka he is best known for winning the bronze medal at the 1997 European Swimming Championships in the men's 4×100 m medley relay, alongside Marek Krawczyk, Marcin Kaczmarek and Bartosz Kizierowski.