Leading retail brokers are trembling over the future of upper Madison Avenue if Barneys closes — as appears likely.

Retail leasing specialists are by nature an optimistic breed. Some are known to say that lots of empty storefronts somehow mean that a particular street is “hot.”

So Newmark Knight Frank Vice-chairman Jeffrey Roseman, really got our attention with a tweet he posted last weekend.

Following reports that the leading bid for the bankrupt fashion store chain by Authentic Brands Group might well result in closing the Madison Avenue flagship, Roseman tweeted: “Madison Avenue without Barneys becomes . . . Third Avenue.”

We know upper Madison Avenue’s in bad shape. We counted 37 vacant stores between East 57th and 77th streets. Many of them are huge, such as locations previously occupied by Calvin Klein and Tom Ford.

But, Jeffrey — Third Avenue, really? That bland, Upper East Side corridor of mid-market chains and lots of empty stores?

“Yes,” he said on Monday. “Barneys defined Madison Avenue. It’s as if Bloomingdale’s left Lexington Avenue or Macy’s left 34th Street.

“CVS and TJ Maxx don’t have the same panache,” he said.

True, Madison “was always a luxury avenue, even before Barneys opened in 1993,” Roseman said.

But in recent years, “It’s really been hit too hard. The rents are too high, and it’s worse than everywhere else.”

Roseman said the avenue wouldn’t be the same without Barneys. He speculated that landlord Ashkenazy Acquisitions would likely “chop it into three or four different pieces” for smaller shops.

Let’s hope Authentic Brands and Ashkenazy make a deal to save Barneys on Madison Avenue — even if it’s only half the size.