Irma Stern's Arab in Black, a painting auctioned to pay Nelson Mandela's legal bills in the 1960s, has sold for £842,500 ($1.2m).
The lot headlined Bonhams' sale of South African art in London on September 9.
When Nelson Mandela was charged with high treason in 1956, a number of artists provided works to be sold to fund his defence.
He was found not guilty in 1961, but given a life sentence in 1964.
The painting was discovered in a London apartment, where it had hung as a noticeboard for many years.
Hannah O'Leary, Bonhams' head of South African art explains: "I was undertaking a routine valuation when I spotted this masterpiece hanging in the kitchen covered in letters, postcards and bills.
"It was a hugely exciting find even before I learned of its political significance."
Congo Woman, another painting by Stern, made £122,500 ($188,401).
Irma Stern is South Africa's most valuable artist. In 2011, a work titled Arab Priest made a record £3m ($4.6m) at Bonhams.
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