Bart Jan-Baptist Marie Brentjens (born 10 October 1968 in Haelen) is a Dutch racing cyclist in mountain biking.

Brentjens won a gold medal for mountain biking in the 1996 Summer Olympics, the first to recognize cross-country mountain biking as an event. He followed this with bronze in the 2004 Summer Olympics. Previously he won gold at the 1995 UCI Mountain Bike & Trials World Championships. In 2007 he became 10th time Dutch National champion

Brentjens competed in Men's category of the Absa Cape Epic in 2005, winning 1st place overall with team mate Roel Paulissen. In 2009 he won 2nd place overall, this time competing alongside Chris Jongewaard. Brentjens then began competing in the Master's category. He won the category in both 2012 with Jan Weevers and in 2014 with Abraao Azevedo. He has completed the marathon stage race ten times to date.

Brentjens, along with Rob Warner, was also a co-commentator for Red Bull TV's official coverage of the UCI Mountain Bike World Cup (XCO). With GCN/Eurosport taking over coverage of UCI Mountain Bike coverage in 2023 Bart is now co-commentating with Kate Mason.

Since November 2016 Bart Brentjens is product manager at American Eagle mountain bikes.