Penn was named best supporting actor for his role as an obsessed military officer in the darkly comic thriller One Battle After Another, one of the movies vying for the top prize of best picture.
He dashed the hopes of Australia's Jacob Elordi, who had been nominated in the supporting actor category for his performance in Frankenstein. Fellow Aussie Rose Byrne is nominated for best actress for her role in the drama If I Had Legs I'd Kick You.
It was the third Oscar for Penn, who frequently skips movie industry awards shows and was not in LA's Dolby Theatre audience.
"Sean Penn couldn't be here, or didn't want to, so I'll accept the award on his behalf," said presenter Kieran Culkin, the 2025 supporting actor winner.
One Battle After Another came into the show the best picture favourite, and it picked up three wins in the first half of the ceremony - best supporting actor, best adapted screenplay and best casting.
As expected, the Netflix sensation KPop Demon Hunters, 2025's most-watched film, won best animated feature.
On Netflix, KPop Demon Hunters became a cultural phenomenon and the streaming platform's biggest hit. It has more than 325 million views and counting.
"This is for Korea and Koreans everywhere," said co-director Maggie Kang.
Another Netflix release, Guillermo del Toro's Frankenstein, picked up a pair of awards for its lavish craft, for costume design and makeup and hairstyling.
Amy Madigan won best supporting actress for her performance in the horror thriller Weapons, a win that came 40 years after the 75-year-old actor was first nominated, in 1986, for Twice in a Lifetime.
Letting out a giant laugh as she hit the stage, Madigan exclaimed, "This is great!" and thanked Weapons director Zach Cregger.
"He just wrote a dream part and he just let me grab it by the throat," Madigan said.
Hosting for the second time, Conan O'Brien began the show alluding to "chaotic and frightening times".
But he argued that the current geopolitical climate made the Oscars all the more resonant as a globally unifying force.
"We pay tribute tonight, not just to film, but to the ideals of global artistry, collaboration, patience, resilience and that rarest of qualities today - optimism," O'Brien said.
Earlier, nominees Jessie Buckley and Byrne led the parade of entertainment luminaries arriving for the unusually open best-picture race that pits vampire hit Sinners against One Battle After Another.
Byrne, who's hoping to win her first Oscar, arrived wearing a strapless black fishtail gown by Dior, with floral embroidery across the bodice and hem.
Another Australian, Nick Cave, is nominated for the title song to Clint Bentley's film Train Dreams, starring Joel Edgerton.
Security for the ceremony was tight as some of the biggest names in Hollywood walked a red carpet decorated with trees to evoke the feeling of a Zen garden.
Organisers said they were working closely with the FBI and Los Angeles police after a federal warning of a possible Iranian threat against California, though authorities have cited no specific or credible danger to the Academy Awards.
This year's awards contest holds an unusually high potential for surprises.
The race for best actor is an especially unpredictable one, pitting Timothee Chalamet against Leonardo DiCaprio and Michael B Jordan.
Chalamet had been considered a frontrunner for his acclaimed performance as a ping-pong hustler in Marty Supreme, but his prospects seemed to dim over an awards-season campaign featuring a streetwear line and a giant blimp and remarks dismissing ballet and opera.
One Battle After Another, starring DiCaprio as a one-time political radical now parenting a teenager, was seen as the frontrunner for best picture after stacking up trophy after trophy at recent ceremonies.
But Sinners, a celebration of blues music and Black culture in the Segregation-era US South starring Jordan, made a late surge with a win this month at the Actor Awards.
with Reuters
29°C