Mathieu Fonteyn (born 14 February 1985) is a Belgian former swimmer, who specialized in freestyle and butterfly events.

[1][2] He represented his nation Belgium at the 2008 Summer Olympics, placing himself among the top 20 swimmers with an illustrious Belgian record in the 200 m butterfly.

[3] Fonteyn trained under the tutelage of the national team's assistant coach Rik Valcke at a local swimming club in Bruges until his abrupt retirement shortly before London 2012 due to sustained injuries.

Leading up to the Games, he placed fourth in the top-eight final at the European Championships in Eindhoven, Netherlands with an Olympic A-cut time of 1:56.86.

[6][7] Fonteyn blasted a new Belgian record of 1:56.65 to obtain the seventh spot and nineteenth overall in the last of six preliminary heats, narrowly missing out the top 16 semifinal by just sixth hundredths of a second (0.06) and trailing all-time Olympian and eventual world record holder Michael Phelps, who competed in the same race as Fonteyn, by nearly three seconds.