Van der Merwe attended Grey College in Bloemfontein before joining the Bloemfontein-based provincial side the. He made his first class debut for them in the 2006 Vodacom Cup – eventually starting eleven matches and playing off the bench on two occasions – and also made his Currie Cup debut in the second half of 2006, three times appearing as a replacement during the competition. In 2007, he got his first taste of Super Rugby, starting two matches for the during the 2007 Super 14 season. He made five appearances in the 2007 Vodacom Cup, but his only Currie Cup involvement in 2007 came in one match on loan at the. He made three appearances for the Cheetahs during the 2008 Super 14 season and also scored his first try in Super Rugby. He played a bigger part in their Currie Cup campaign in 2008, making seven appearances.
In 2009, Van der Merwe moved to Pretoria to join the. He immediately established himself as a first-choice player for them, making eleven starts and two substitute appearances in the 2009 Currie Cup Premier Division. He also featured for the in the 2010 Super 14 season before being called up to the South African national teamfor the first time. He became an important part of the Bulls side over the next few years, making 75 appearances for the side in Super Rugby competitions between 2010 and 2015. He also featured in 39 first class matches for the Blue Bulls between 2009 and 2012, with national team call-ups and injury preventing him from playing for them in 2013 and 2014.
Clermont
Shortly before the 2015 Super Rugby season, the announced that Van der Merwe would leave after the 2015 Currie Cup Premier Division to take up a contract with FrenchTop 14 side. He was later given an early release from his contract and made the move to Clermont during the Currie Cup campaign.
Representative rugby
On 12 June 2010, he played his first test for the South African national team, the Springboks, against France. He missed out on selection to the 2011 Rugby World Cup, but established himself as a regular for the team over the next few years, appearing in 36 test matches prior to his move to Clermont in 2015.
Personal
His father Flippie was also a rugby player, making six appearances for South Africa between 1981 and 1989.