The Upper West Side suffered its third water-main break in a month Friday morning.

A 20-inch pipe — dating back to 1912 — ruptured around 8:30 a.m. on Broadway between 111th and 112th streets and quickly flooded the block, authorities said. NYPD soon closed southbound traffic on Broadway, cops said.

Firefighters responded to the scene and reported that a few building basements had been flooded, the FDNY said, including the basement of a Citibank on Broadway.

“Our entire downstairs is flooded,” one worker said, noting that the whole basement is soaked in water.

“It’s terrible.”

The water was shut off at 10:10 a.m., according to the city Department of Environmental Protection.

Pictures and videos of the scene show the asphalt bubbling and beginning to crack open in spots. Pedestrians were temporarily directed to walk on the east side of the avenue.

“As you can see, the pavement buckled, so we’re working on removing the pavement, getting to the water main and restoring it,” said DEP Commissioner Vincent Sapienza at the scene.

“Once that’s done, we’ll excavate, we’ll get down to our 20-inch water main, replace the section that broke, and get it back in action.” All in all, “The street restoration might take a couple [of] days.”

The flooding was contained to the southbound side of Broadway, but the torrent was heard down the block.

“It sounded like a bomb” said Louie Valentin, a doorman at the first building off Broadway on 111th Street. “Boom! It was already coming down… the whole street. It was like a river.

“I never seen anything like it,” he said, “The water was really, really dirty.”

On Jan. 19, only a few blocks away on Central Park West near 103rd Street, a water main broke at 8 a.m. flooding a three-block radius and shutting down traffic and subways for hours.

A few days earlier, on Jan. 13, another water main broke near Lincoln Center, turning streets into rivers throughout neighborhood — flooding parking garages, the Nos. 1, 2 and 3 subways and several basements.