Nearly 1,000 box jellyfish have been counted on Oahu's south shore as of 8:30 a.m. Thursday, according to Ocean Safety officials.
Lifeguards at Waikiki and Ala Moana beaches have posted signs warning the public.
KITV spoke with at least two visitors who were stung at Queen's Surf Beach.
"There's one on my right arm. There's one on my abs, and there's one on my left legs," said David Li from New York.
Volunteers also collected about a dozen box jellyfish on Oahu's Leeward Coast. Officials say the influx is usually in the morning.