Ante Juricev-Mikulin, 22, received the injuries fighting a blaze in the national park on the central Adriatic island of Kornat on August 30.
It was the worst incident of its kind in the country.
Six firefighters, including both professionals and volunteers, were killed on the spot, while another six, including Juricev-Mikulin and two 17-year-olds, have now died in hospital.
Meanwhile an official from a volunteer fire service which lost four members in the fire committed suicide on Sunday, police said. 
The secretary of the volunteer fire department from the small coastal town of Vodice, Gordan Alfirev, hanged himself.
Police did not elaborate but according to local media reports he could not cope with the tragedy. 
According to preliminary results of a investigation into the deaths, the firemen became cut off when the wind suddenly changed direction and caused the flames to flare up.
The findings, revealed earlier this week, also showed that two firefighting commanders, one of whom died in the blaze, failed to react adequately to prevent the disaster. The other commander was detained earlier this month. 
Police previously arrested a national park receptionist suspected of starting the fire by throwing a cigarette butt into the grass.
The tragedy was most likely caused by a "combination of unfortunate circumstances," police chief Marijan Benko said.
Since June, around 13,000 hectares (32,000 acres) of land have been devastated by some 900 fires mainly along Croatia's coastline, according to the authorities. 
The fires were mainly the result of hot and dry weather, although police have brought charges against about 30 people for arson, the interior ministry said.