"I'm going to stop at the end of the season, when your body tells you stop, you have to know how to listen to it and be honest", the lock, who turns 34 in June, said.
Van der Merwe joined Clermont in 2015 and won the Top 14 with the club in 2017. He will play his 80th match for the club against Perpignan on Saturday.
Clermont are second in the table and, with three rounds to go, are on course for a bye in the first round of the playoffs. They are also in the European Challenge Cup final and face La Rochelle, in Newcastle on May 10.
"We have to maintain the same level for the exciting end of the season ahead," said Van der Merwe, who is returning after a five-week ban for a high tackle.
Before hanging up his boots "two trophies would be nice," he said.
Van der Merwe played 37 times for South Africa and won a Super Rugby title with the Bulls, "but I finished my education here," he said on Thursday.
He said he hoped to stay in Clermont with his family.