Cyndi Lauper's Son Declyn Arrested for Being in Stolen Car

Cyndi Lauper's son may have just wanted to have fun ... but cops say he broke the law when they found him inside a stolen vehicle.

The NYPD tells TMZ ... 24-year-old Declyn Lauper -- a rapper with more than 2 million Instagram followers -- was taken into custody early Thursday morning after cops say they found him in the driver's seat of an illegally double-parked Mercedes.

We're told police ran the plates as part of their investigation, and the plates came back as registered to a different car. After checking the VIN number, they say the Mercedes he was in had been reported stolen 2 years ago.

Declyn was taken to a nearby police station and booked for felony unauthorized use of a vehicle.

The artist, who has a music career under the name Dex Lauper, posted about the incident alongside some choppy videos of the supposed arrest.