Victoria Beckham is flexing her power ... not just on the runways for New York Fashion Week, but by showing that's she's got a firm grip on her hubby David.

While out and about in NYC, Posh jokingly posed with a billboard of David wearing nothing but his undies. Love her sense of humor!

David posted the photo with the following: "So proud of my wife taking #NYFW by the balls."

Don't mess with VBecks!