A customer spent nearly 10 hours locked inside a giant vault in Manhattan’s Diamond District before it suddenly sprung open early Wednesday, allowing him to walk out unscathed.

The customer, an unidentified 23-year-old man, was accessing his lock box inside the room-sized steel vault Tuesday night when a timed mechanism suddenly locked him in, officials said.

Police were called around 8:45 p.m. to help try to free him from the DGA Securities’ vault inside 580 Fifth Avenue, a large commercial space known as the World Diamond Tower that is home to numerous jewelry businesses.

FDNY and NYPD personnel then spent nearly 10 hours working through the night trying to break into the vault, police told The Post.

They got through about 30 inches of concrete and down to the metal plating of the vault, which is also reinforced with steel bars, FDNY Deputy Chief John Sarrocco told reporters.

Throughout, crews were able to chat to the trapped man by phone and see him on cameras inside the vault, which at 20 feet by 40 feet was assumed to have plenty of oxygen, officials said.

Then “the vault opened on its own — it’s on a timing mechanism — and he was just released from the vault,” Sarrocco said. 

The trapped man was able to walk out around 6:15 a.m., with images online showing a man with a backpack soon rushing away from the scene.

“He’s fine,” Sarrocco said, while police stressed that no criminality is suspected.

The vault company, DGA Securities, is used by businesses in the famed jewelry area, with numerous secure viewing areas for big-spending customers.

The company’s website says the vault offers “the security, services and tools you need to conduct business in today’s Diamond District.”

“The vault is protected by DGA’s network of security and surveillance systems monitored 24 hours a day, 365 days a year,” it says.

“Providing unparalleled protection along with impeccable office amenities, this is your one place to have it all.”