Liu Siying died Saturday night of sudden heart troubles at Jishuitan Hospital, where she had been receiving treatment since she and four others set themselves ablaze on Jan. 23, Chinese Central Television said.
One of the four, Liu's mother, died that day on the square. The three others are still in the hospital's burn unit.
The Chinese government has said the five were members of the Falun Gong spiritual group, which it banned 19 months ago as a threat to social order and communist rule.
Falun Gong has denied that the five were members, saying its teachings do not condone suicide.
Beijing seized on the group suicide on the traditional Chinese New Year's Eve to drive home its message that Falun Gong is an evil cult that callously pushes its members to acts of self-destruction.
Gruesome images of the five ablaze or their blackened bodies lying on Tiananmen's gray flagstones were beamed on national television.
Government propagandists focused in particular on Liu, showing photos of a smiling, pretty girl in a school uniform and then footage of her writhing charred face crying out for her mother.
The campaign has apparently been effective, creating genuine revulsion for the sect among Chinese. After weeks in early January when state media said hundreds of Falun Gong followers were being rounded up daily on Tiananmen Square, demonstrations by the group have also seemed suddenly to decline.
Falun Gong attracted millions in the 1990s with its mix of traditional Chinese religion, health exercises and the teachings of founder Li Hongzhi, a former government grain clerk now in the United States.
It was outlawed in July 1999 after the group surprised Chinese officials when more than 10,000 members surrounded the leadership's living compound in Beijing in a demonstration to demand official recognition.