It happened around 4:35 p.m. in the area of 21st Avenue beach in Seaside Park, Ocean County.
The Berkeley Township Police Department said four lifeguards and four other beachgoers were struck by lightning. One of the lifeguards died in the incident.
"This young person was out there every day protecting the lives of others. Our lifeguard teams, like so many along the shore, develop special connections with our community throughout the summer, which makes this loss even greater," said Berkeley Township Mayor Carmen Amato.
Police say the eight other victims were transported to area hospitals, but there was no immediate word on their conditions.
"Our hearts are with the family and friends of the young lifeguard killed by today's horrific lightning strike on the beach in Berkeley Township, and we pray for a full recovery for those injured," said New Jersey Gov. Phil Murphy Monday night.
The lifeguard has not been identified.
This is the second young New Jersey lifeguard that has died in recent weeks. On August 19, Norman Inferrera, a rookie lifeguard in Cape May, was rowing a lifeguard boat when a wave knocked him unconscious. He died at the hospital days after the accident.
Cape May officials are now re-examining the use of surfboats following Inferrera's death. The Press of Atlantic City reports that Cape May lifeguards won't train or use the boats for the rest of the year amid a re-examination of their role.