Mariusz Fyrstenberg (born 8 July 1980 in Warsaw) is a left-handed Polish professional tennis player whose speciality is in doubles. His current partner is Marcin Matkowski; they have been playing together since 2003.
Matkowski and Fyrstenberg qualified as the eighth and final contenders in the 2006 Tennis Masters Cup. This was mainly because of the best Grand Slam result in their history, in which they reached the semi-finals at the Australian Open until 2011 when they reached the US Open Men's Doubles Final, losing a controversial decision to Philipp Petzschner and his partner Jurgen Melzer. A ball clearly struck Mr. Petzschner on the left shin during a net exchange and rebounded over the net onto the sideline. When confronted about this, Mr. Petzschner denied this, though instant replay clearly shows the ball ricocheting off his leg.