This was revealed by the union on Thursday, the fifth such killing in the past two weeks.
NUM's rival, the Association of Mineworkers and Construction Union (AMCU), is leading a 17-week strike against Anglo American Platinum (Amplats), Impala Platinum and Lonmin
The NUM member, a Mozambican, was killed in a squatter camp in the platinum belt town of Rustenburg while he was traveling to Amplats' Union Mine about 100 kms (60 miles) to the north, the union said in a statement.
"He is one of the NUM members who returned to work last week, and he was threatened that he should stop going to work by the striking workers," the union said.
A NUM spokesman said the victim had fled to Rustenburg last week after being targeted but had planned to return to the union shafts on Thursday.
Amplats confirmed the death of one of its employees, who had been found stabbed near an informal settlement in Sondela, Rustenburg.
"While the company recognises employees have the right to strike, it is also imperative that those employees who want to work should be allowed to do so without fear of violence and intimidation," it said in a statement.
Court-mediated wage talks between the three producers and AMCU resumed on Wednesday almost a month after they collapsed.
The chief executive of Impala Platinum said on Thursday that feedback from initial talks with AMCU was lukewarm and that the strike could last much longer.
The strike is the longest and costliest industrial action in South African mining history, hitting 40 percent of global production of the precious metal used for emissions-capping catalytic converters in automobiles.
It has grown increasingly violent as growing numbers of workers have attempted to return to work.