The U.S. Forest Service said the death happened Thursday but did not release additional details.
The cause of death is under investigation. The name of the firefighter is being withheld until family members are notified.
RELATED: Firefighter dies battling El Dorado Fire sparked by gender reveal party pyrotechnics
"Our deepest sympathies are with the family, friends and fellow firefighters during this time," the agency said in a statement.
The El Dorado Fire, which erupted Sept. 5 near Yucaipa, was 66% contained as of Friday morning. Crews are trying to stop flames from crossing Highway 38.
Officials said the blaze was caused by a smoke-generating pyrotechnic device used during a gender reveal party.
RELATED: El Dorado Fire near Yucaipa sparked by pyrotechnic device at gender reveal party, officials say
Wildfires in California this year are blamed for at least 25 deaths.
The Associated Press contributed to this report.