Chrissy Teigen's standing up for Jeff Bezos' fiancée Lauren Sanchez -- this after a notorious restaurateur unleashed a barrage of venom toward ... for no real reason at all.

Balthazar owner Keith McNally randomly threw up a bunch of IG pics of Lauren Monday with JB at various high-profile events Monday -- adding a totally out-of-line caption that touched on her appearance ... and what he considers her aura too.

He says, "Does anybody else find Jeff Bezos' New wife [sic] – Lauren Sanchez – ABSOLUTELY REVOLTING? What an ugly and **king SMUG – LOOKING couple they make. Is this what having 1000 Billion dollars does to people?"

The post was so ruthless, it seems, that Chrissy hopped into his comment section to defend Lauren, writing ... "She's actually incredibly dynamic, accomplished, and kind, and everyone who knows her would say the same."

MSNBC's Stephanie Ruhle clapped back too ... calling his comment flat-out mean, and adding it was people like him who sucked the joy outta social media. In fact, a quick scroll down the comments on his post, you'll see most everyone's team Lauren and calling him out.

It's a total mystery why McNally decided to go off about Lauren's looks ... and whether they even personally know each other in the first place.

One thing's for sure, this isn’t Keith’s first rodeo stirring up headlines with his outbursts. Back in 2022, he famously called James Corden a "cretin" for allegedly mistreating his staff at Balthazar in NYC ... we all remember that saga well.

Waiting for your permission to load the Instagram Media.

Either folks are really getting on McNally's last nerve, or he's just thirsty. You can decide.