 Marx earned his first start for the Springboks on 5 November 2016, putting a 51-minute performance into a 31-31 draw against the Barbarians Club, prior to being replaced by Bongi Mbonambi.After a series of outstanding performances for the Lions, Marx became a regular starter for South Africa in 2017.On 10 June 2017, he was named man of the match for his performance in the first of three tests against the French.