It’s springtime for Banksy in Gramercy.

The elusive graffiti artist’s latest New York installment is a “vandalized” painting — into which the graffiti artist inserted the image of a Nazi officer sitting on a bench looking out over a pastoral scene — that’s expected to spark a wild online bidding frenzy, with a starting auction price of $76,000.

One self-described Banksy authority speculated that the artwork will fetch “a million dollars, at least.”

The oil-on-oil painting appeared in the window of the Housing Works Gramercy thrift shop on East 23rd Street Tuesday, as Banksy headed into the homestretch of his “Better Out Than In” New York tagging tour.

All of the proceeds will go to charity group, which focuses on homelessness and HIV/AIDS issues.

“This is one of the coolest things that has happened to us in a long time,” Housing Works spokeswoman Rebecca Edmondson told The Post.

“It’s such a great gesture, what he did today is really special. He may be controversial, but there is no controversy in the fact that it goes to something good.”

The special two-day auction will end October 31, the same day Bansky’s month-long city “residency” is slated to end.

Banksy titled the new piece “’The banality of the banality of evil’ Oil on oil canvas, 2013,” describing it on his website as “A thrift store painting vandalized then re-donated to the thrift store.”

It’s believed that Banksy bought the original painting at the Housing Works shop, reworked it and then brought it back.

“I don’t know all the details, it was all done very secretively. But someone came in and dropped it off as an anonymous donation,” Edmondson said.

Housing Works got a call Tuesday authenticating the work as Banksy’s, she said.

The painting and store with will be guarded 24/7 through the end of the auction, Edmondson said.

But there was no security in sight when Store clerk Alex Vaughn opened the thrift shop this morning – because nobody knew what it was.

“I open the store at 9 AM,” Vaughn said. “The piece was already in the window. No security. We opened to the public at 10 AM, and it was up all day.

“I didn’t think of it, but I’m the only one who opens and it was there. Then in the afternoon crowds started coming, and we all sort of figured it out.”

The Brit-based artist signed his name under that of the original artist, “K. Sager.”

The Banksy auction started Tuesday night at www.biddingforgood.com, Edmondson said. There will be a link to the painting at Housing Works’ online auction site, and details on Twitter at @HWthrifts.

Housing Works’ thrift shops help fund the advocacy group’s HIV/AID and homelessness initiatives.

“Banksy’s been doing crude oil paintings for years,” said Jon Satin, American moderator of banksy.info, the self-proclaimed online authority on the graffiti artist. “He’ll take a traditional piece, and then spoof it.”

While the Nazi depiction might scare away some prospective dealers, Satin nonetheless thinks the piece “will go for a million dollars, at least.”

Banksy’s “banality of evil” tag appears to be a reference to a 1963 book, “Eichmann in Jerusalem: A Report on the Banality of Evil” by political theorist Hannah Arendt, documenting the post-WWII trial of the Nazi SS leader who was a major architect of the Holocaust.