Standing beside his bandmates, bass player Flea announced the show in Irvine, California, wouldn't go on.
Singer Anthony Kiedis, 53, was heading to the hospital, Flea told the crowd.
"We are devastated about it," the bass player said. "We live to rock ... but unfortunately there's a medical thing that happened and he needed to deal with it."
In a statement Sunday
, the band announced that "complications from the intestinal flu" led to Kiedis' hospitalization.
"Anthony is expected to make a full recovery soon," the statement said.
But it will be some time before the Red Hot Chili Peppers are back onstage. They're also postponing their scheduled May 17 show in Burbank, California, the statement said.
'We are so sorry'
The news quickly spread on social media Saturday night as concertgoers at the show shared videos of the last-minute announcement.
"We're unable to play tonight," Flea told the crowd in Irvine. "We will be playing for you very soon, and we will be playing for you like the feral animals full of love that we are. And we are so sorry."
Later Saturday night, drummer Chad Smith tweeted, "NO one's more disappointed than us that we couldn't perform tonight. Sending love and a speedy recovery to my brother Anthony."
Summer tour scheduled
Formed in the 1980s, the Red Hot Chili Peppers pioneered a bold style of rock infused with funk and rap. The enduring group was inducted into the Rock and Roll Hall of Fame
They were among more than a dozen bands who'd been scheduled to perform Saturday at an annual concert put on by KROQ, one of Los Angeles' most popular radio stations.
The station tweeted shortly after the band's set time that they wouldn't be performing "due to illness."
It's unclear whether the band will be calling off any other performances. After the now-postponed Burbank show, their next scheduled concert is May 23 in Columbus, Ohio. They're also scheduled to tour Europe this summer, according to the band's website