The cagey street artist surprised the Housing Works Thrift Shop in Gramercy on Tuesday by returning a pastoral oil painting he had bought for $50, after he added a Nazi soldier sporting a swastika armband.

Store officials wasted no time in auctioning the piece on BiddingForGood.com. A bidding war started at $74,000 and stood at $201,200 as the clock struck midnight Tuesday night. No telling what the price will be when the online auction ends at 8 p.m. Thursday.

Housing Works plans to use its windfall to fund its mission of fighting homelessness and AIDS.

The crowd at Housing Works Thrift Shop in New York takes pictures of the latest Banksy work.

Josh Haskins, the store assistant manager, said the painting, originally done by an obscure artist named K. Sager, mysteriously appeared at the shop Tuesday morning.

“Someone showed up at the counter and told a volunteer to get the manager, saying, ‘A valuable piece of art is here,’” said Haskins. “He then just walked away.

“I took one look at it and I knew what it was,” said Haskins, 27, of Brooklyn. “I’ve been into Banksy since I was in college.”

Banksy fans gather to take snapshots of ‘The banality of the banality of evil,’ the artist’s latest work on sale at the Housing Works Thrift Shop.

Rebecca Edmondson, spokeswoman for Housing Works, said proceeds from the painting would help thousands.

“Housing Works is thrilled to receive such a generous donation from Banksy,” Edmondson said in a statement. “It means a lot to our organization that the artist is using his time in New York to give back to the very community that has been captivated by his every move.

“Proceeds from this one of a kind piece will do so much good for the thousands of men, women and children dealing with the dual crisis of homelessness and AIDS that still plagues our city streets.”

Hung just so above a couch in the Housing Works Thrift Shop, art collectors should have no trouble imagining the oil painting modified by Banksy in their own living rooms.

To the painting, Banksy added the Nazi soldier sitting on a wooden bench and staring contemplatively at the original artist’s fall landscape of a peaceful river flowing from a snowy mountain.

The famed British graffiti master signed the painting under Sager’s signature.

“A thrift store painting vandalized then re-donated to the thrift store,” Banksy wrote on his website.

Not everyone can afford $153,601 — possibly more — for a painting, even one transformed by famed street artist Banksy, so they’re taking photos of it instead.

He titled the doctored painting: “The banality of the banality of evil.”

While Banksy didn’t offer an explanation, the work is reminiscent of a 1969 episode of Rod Serling’s TV show “Night Gallery.” In an episode titled “The Escape Route,” a Nazi war criminal, haunted by past demons and confronted by a Holocaust survivor, finds solace in a serene museum painting.

The E. 23rd St. store immediately drew a crowd of photographers and Banksy fans after the shop hung the framed 3-by-2-foot painting in the front window above a checkered pink couch.

The donation to Housing Works came just days after Banksy angered loved ones of 9/11 victims by slamming the design of 1 World Trade Center as “vanilla” and a symbol that “proclaims the terrorists won.”

Banksy has been on a month long residency in New York, tagging buildings with provocative graffiti and erecting installations in vacant lots and even on trucks roving the city.