Hanauma Bay was closed for three hours Saturday as police investigated a suspicious object found in the water.
According to the Honolulu Fire Department, around 10:40 a.m. a lifeguard conducting training near an area known as Witches' Brew spotted the device and brought it to shore.
As a precaution, officials evacuated dozens of beachgoers.
"We weren't really sure what was happening," said Antonio Galvao, who is visiting from Brazil. "Someone from the lifeguard tower called us in his megaphone saying everyone had to evacuate the beach."
"People were worried if it was a package or a shark, we didn't know what was going on," said another visitor Phoebe Taylor. "We didn't see anything in the water we just saw everyone coming out of the water"
Honolulu Police Department's bomb squad was called to the scene.
Visitors trying to get in were turned away and traffic along Kalanianaole Highway was backed up for miles.
Fire officials confirmed the suspicious item found was an extended marine flare.
No one was injured and Hanauma Bay was reopened to the public by 2 p.m.