Luann De Lesseps ain't a housewife today ... she's in total vacation mode instead -- stripping down to a bikini on a beach in Mexico.

The 'Real Housewives of New York' star went out for some surf and sand in Tulum earlier this week ... whipping out a black 2-piece bikini and a matching hat while slinging a beach bag over her shoulder.

She stops to take a few selfies with her pals ... popping a hip while snuggled up to a pair of shirtless dudes.

Luann doesn't get into the water in these pics ... but, she's certainly ready to sun herself in the sand while enjoying much-needed R&R.

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De Lesseps is posting about her trip too ... sharing photos from another day when she's lying back in a beach chair -- wearing a bright blue bikini instead of her tiny black number.

De Lesseps has a couple weeks to herself before her cabaret show "Countess Cabaret" beginning on February 1 and running periodically through the beginning of June.

Countess Luann will definitely have a tan for those early shows ... 'cause she's letting it all hang out while getting some sun!