Kateryna Handzyuk, 33, suffered burns over 40% of her body and severe eye damage as a result of the assault on 31 July in the southern city of Kherson.
Ms Handzyuk, who was also a Kherson city council member, had undergone 11 operations in the capital Kiev.
President Petro Poroshenko on Sunday called for the killers to be punished. Five suspects are already in custody.
The exact cause of Ms Handzyuk's death was not immediately known, but reports in local media suggested it was a blood clot.
In September, the activist - who also campaigned against Russian-backed separatism - posted a video in which she urged Ukrainians to fight rampant corruption.
"I know I look bad now. But at least I'm being treated," she said. "And I definitely know that I look much better than justice in Ukraine. Because nobody is treating it."
The death of the activist on Sunday triggered an avalanche of reactions on social media in Ukraine, with many users expressing shock.