The Thomas fire slowly eclipsed previous record-setting blazes, finally overtaking the 2003 Cedar fire in San Diego County, which burned 273,246 acres.
It follows a series of deadly fires in the state’s wine country in October that burned more than 10,000 homes and killed more than 40 people.
The Thomas fire has destroyed more than 1,000 buildings and claimed the life of one firefighter – Cory Iverson, a father of one from San Diego whose wife was expecting another child.
It is now moving slower because of rain and less wind.
State fire agency Cal Fire says the Thomas fire is now 65% contained and expected to continue to slow. Controlled burns by firefighters may cause some temporary expansion, it said.
Seven of California’s 10 largest fires on record have occurred since 2000. Two were in the 1970s and the earliest was in 1932 – the Matilija fire which, like the Thomas fire, burned through Ventura County.
California Governor Jerry Brown has said that devastating wildfires fuelled by climate change are “the new normal”.
He said vast fires, such as the ones that have ravaged southern California in recent days, “could happen every year or every few years”.
“We’re facing a new reality in this state,” the governor said.
Jerry Brown made the comments after surveying the damage in Ventura County, north of Los Angeles.
Thousands of firefighters have been battling the fires since December 4.
Jerry Brown, a Democrat who has attacked the Trump administration’s stance on climate change, said: “We’re facing a new reality in this state, where fires threaten people’s lives, their properties, their neighborhoods, and of course billions and billions of dollars.
“With climate change, some scientists are saying southern California is literally burning up.”
The largest wildfire – known as the Thomas Fire – burned close to 150,000 acres, an area of land roughly the size of Chicago, Reuters reported.
On December 9, firefighters began to make progress in containing the blaze.