City Council unanimously approved the curfew following a request from the police department, which says there have been a few incidents this summer involving gatherings that became disorderly.
Several arrests have stemmed from fights and underage drinking.
Mayor Zachary Mullock said police began noticing a trend in which minors from neighboring towns with existing curfews were coming to Cape May later at night to continue partying.
"We really wanted to make sure we didn't become the afterparty to some of the other towns that have already made this move," Mullock said.
The mayor said there have only been a handful of incidents so far, but the city wanted to address the issue before it got out of control.
Under the new rules, anyone under 18 is prohibited from being on the beaches after 8 p.m. and from being in public places after 10:30 p.m., unless accompanied by an adult.
Backpacks, bags and other containers are also prohibited on the promenade beginning at 8 p.m.
"After 10:30, we all know it's not normally the best stuff that happens then so, to be honest - as a parent myself - I think there's a good thing," Mullock said.
Visitors said they understood the reasoning behind the new restrictions.
"The thing with Cape May especially, this place kinda shuts down at what - 10, 10:30. So I mean, people are going to bed, I kinda understand it," said Jack McCune of Yardley.
"I think that's smart. That's planning ahead," said John Christiansen from Staten Island. "It's really just a few bad kids that cause trouble, so once everyone finds out what they need to do then it's good. What happened to game night?"
"Well, if there's a problem, they're addressing it," Mary Kelly from Lawrenceville said. "I haven't seen anything or noticed anything. But perhaps they need to be proactive and take care of it sooner rather than later."
The new rules took effect for the first time Thursday night.
"Frankly, last night, they were out here and the reports I received is that it worked and we just had families out here enjoying a pleasant evening," Mullock said.
The curfew includes exceptions for juveniles who are going to or from work.