Zingiswa Losi, 42, was nominated unopposed at an annual meeting of the Congress of South African Trade Unions (Cosatu) in Johannesburg’s northern Midrand suburb.
The former soldier served in the South African army for three years before going to work at the Ford car plant in the southern city of Port Elizabeth.
Her workers’ rights activism began while she was in the military, according to Cosatu which has 1.6 million members.
Cosatu plays a key role in South African politics and provided logistical support to the anti-apartheid struggle.
Since the advent of democracy in 1994, it has been an important source of grassroots support for the ruling African National Congress party (ANC), although it has been riven by factionalism.
South African President Cyril Ramaphosa congratulated Losi, saying her election “affirms women – and black women in particular – as political activists and social reformers, leaders in their own right and agents of their own liberation”.
“I am confident that in this eminent role, Ms Losi will serve the interests of workers with the passion and dedication she has displayed throughout her career as an activist and political leader,” Ramaphosa said.
“This election is an important step forward”.