After Luann de Lesseps triumphantly announced Thursday night that she’ll be appearing in “Chicago” on Broadway, reps for the reality star tell us she is sadly mistaken.

While her “Real Housewives of New York” co-stars were casting doubt on her singing chops during its reunion show, de Lesseps, 54, she told them, “I want to tell you all right now, that for someone who can’t sing, I just got cast in ‘Chicago’ on Broadway.”

But on Friday her rep told us she’d jumped the gun. “Due to scheduling conflicts, between her [Bravo show], her jewelry line and her successful cabaret tour [with ‘The Countess and Friends’] Luann will not be joining the cast of ‘Chicago’ at this time.”

De Lesseps — who has been trying to land a role in the long-running musical for nearly a decade now — had been telling pals as recently as six weeks ago that she’d been cast as Mama Morton.

And a source told us production had been in talks with de Lesseps but had ultimately decided to go in a different direction. De Lesseps has had a recent run of unpredictable behavior (over the last couple of years, she’s been arrested for a drunken rampage, taken into custody again for allegedly violating her parole, and she’s gone to rehab twice).

When Page Six contacted the show about the scuttlebutt a couple of weeks ago, they told us there were no plans for her to appear. A production spokesperson told us at the time, “Luann de Lesseps is not confirmed to join the cast of ‘Chicago’ at this time.”

Apparently, de Lesseps didn’t get the memo. A source said that de Lesseps had jumped the gun by making the unplanned announcement on the reunion, perhaps because she was feeling defensive while under attack from her fellow “Real Housewives.”