A Japanese pop artist tipsily scribbled on the walls of an East Village dive bar a decade ago — and the mural could now be worth millions.

The value of Yoshitomo Nara’s doodles at Alphabet City hipster mecca Niagara skyrocketed after his painting “Knife Behind Back” sold for $25 million at Sotheby’s Hong Kong on Sunday, shattering his previous sale record.

“[The mural’s] been just hanging out for 10 years,” said Tim Blum, who has represented the Japan-based artist since the 1990s. “Nobody paid it too much attention until now.”

Experts recently told Artnet News that Nara’s 3-foot-high and 8-foot-long mural of his signature doe-eyed cartoonish characters could be worth as much as $5 million. There are also some drawings of the figures near the bathrooms at the back of the bar.

“Its the kind thing auction houses would love to sell: it’s storied, it’s a site-specific moment, it has a history …” said Sharon Hurowitz, an independent curator and adviser.

“I think the sky is the limit … It’s something people would grab onto and be interested in. You could see another sensational price,” she told The Post.

While the doodles are now worth a pretty penny — that wasn’t the artist’s intention, when, after a couple of drinks, on Feb. 26, 2009, he pulled out a marker and started drawing.

“He felt particularly free that night,” Blum recalled to The Post. “One thing led to another, and it was quite unscripted, nobody got in the way.”

What’s even more memorable about the evening is Nara being busted at the First Avenue L train stop around 3:30 a.m. for drawing a smiley face on the subway station wall.

“The thing we remember most is that he got arrested and had to be bailed out,” Blum said — especially since the bust happened the night before Nara’s opening at the Marianne Boesky Gallery in Chelsea.

A bartender at the dive, Jeremy, 28, said Thursday there are no plans to take down the work, which is covered in plexiglass, and sell it.

“They’re here as long as this place is here,” he said.

One 55-year-old regular, nursing a $5 glass of tequila, said she was “shocked” that the bar’s “DIY decor” turned out to be an “artwork worth a lot of money.”

“It’s like there’s only this thin layer of plastic separating us from art history,” she said. “It’s a doodle on the wall. But to other people it’s a work of art.”

Though the scribbles are signed and dated by Nara, there’s nothing else indicating that they’re by one of the most expensive living artists.

“It’s this totally amazing, classic New York style, where they don’t overly promote it at all, it’s just kind of … there,” Blum said.

Nara, who remembers making the mural, wouldn’t like to see it sold, Blum said.

“He wouldn’t deny that he painted it,” Blum said. “But he wouldn’t go so far as to partake in the monetization of a crazy evening in New York City.”

Additional reporting by Alex Taylor