Piotr Marcin Gacek (born 16 September 1978) is a former Polish volleyball player, a member of Poland men's national volleyball team in 2005–2010 and 2015–2016, European Champion 2009, silver medalist of the World Championship 2006, two–time Polish Champion (2009, 2010).
Gacek was born in Grodków, Poland. Married twice, his first wife was Agata. On September 27, 2008 he married Karolina. In August 2010 his wife gave birth to their daughter Zofia.
His first professional club was SKS Mechanik Nysa and then he played for PGE Skra Bełchatów. With the club from Bełchatów he achieved the titles of Polish Champion (2009, 2010). In 2010-2014 he played for Polish club ZAKSA Kędzierzyn-Koźle. He won with this club from Kędzierzyn-Koźle the silver medal of CEV Cup 2011, silver (2011), bronze medal (2012) of Polish Championships and two Polish Cups in 2013 and 2014. In 2014 he moved to LOTOS Trefl Gdańsk. On April 19, 2015 LOTOS Trefl Gdańsk, including Gacek, achieved the Polish Cup 2015. Then he won silver medal of the Polish Championship. In May 2015 he signed a two-year contract with LOTOS Trefl Gdańsk.