Doctors treating Choi Hyun-Yul, 81, said his condition had deteriorated rapidly due to blood poisoning.
Choi set himself alight during a rally by some 1,000 protestors outside the Japanese embassy in Seoul on August 12, ahead of the 70th anniversary of the end of Japan's 1910-45 colonial rule over the Korean peninsula.
Choi had been a regular at the monthly protests outside the embassy to demand reparations for so-called "comfort women" -- an extremely emotive issue in South Korea where fewer than 50 of the thousands of women coerced into prostitution remain alive.
Self-immolation is not that rare a form of protest in South Korea and was particularly common during the pro-democracy movement of the 1980s and early-90s, when a number of student activists set themselves on fire during public demonstrations.
The last such protest outside the Japanese embassy was in 2005, when a 54-year-old man set himself on fire during a protest over Japan's claim to a set of South Korean-controlled islets in the East Sea (Sea of Japan).