The cause and severity of Kiedis' illness was not immediately clear.
Fellow band member Flea made the announcement from the stage at Irvine Meadows Amphitheater. The band was scheduled to close out KROQ's annual Weenie Roast concert.
"We are devastated about it, we're really sad," Flea said while apologizing to the crowd. "Unfortunately there's a medical thing that happened and he needed to deal with it."
A representative for KROQ told Billboard magazine an ambulance took Kiedis to the hospital with "extreme stomach pain."
Red Hot Chili Peppers drummer Chad Smith shared his well wishes for Kiedis on social media and expressed the band's regret over canceling their performance.
Weezer closed out the show instead.
Kiedis has fronted the Red Hot Chili Peppers since the band's formation in 1983. The group is scheduled to play next on Tuesday in Burbank, California.
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