Hackers briefly shut down the official city website Tuesday.
The city's information technology department says NYC.gov was the target of what's called a "denial of service attack" Tuesday.
The site was down for two and a half hours.
Officials said there was no damage to the site or any of the city's underlying systems.
They are now working with the NYPD to find out who was behind the attack.
NYC.gov serves more than 100,000 visitors a day.