The agency announced the shutdown of the two-lane highway at the Pacific Coast Highway intersection shortly before 8:30 a.m.
No injuries or vehicle damage were immediately reported. Photos released by Caltrans showed debris and massive rocks in the roadway.
It was unclear how long it would take to remove the boulders, mud and tree branches.
Crews were also checking the north end of Highway 23 in the Westlake Village area, officials said.
Decker Canyon/Malibu: State Route 23 is closed on the south end at State Route 1 (PCH) due to mud & rock slides. Crews are checking the north end of SR-23 in Westlake. #slides #SR23 pic.twitter.com/cTSARdcxhf— Caltrans District 7 (@CaltransDist7) December 30, 2021