Law Roach says he didn't fly in style from New York to Los Angeles on Monday ... he had to sit up the whole way like a normie -- instead of getting to lie down.

The celebrity stylist took to X to air his grievances ... calling out Delta because he says he was promised a "lay flat seat" and didn't get one -- a huge disappointment for him, he notes.

Roach also flexed his credentials online ... calling himself a Delta 360° member -- an exclusive invitation-only club for the airline's loyalists -- as well as a million-miler.

Basically, Roach is a loyal customer ... and he's pissed at the subpar accommodations.

As you can imagine, Roach isn't getting a ton of support from the regular folk on social media. Many are pointing out they've been crammed in coach their entire lives ... and Roach probably could've handled this by calling the company's customer service hotline to complain instead of immediately jumping on X.

Roach is just the latest celeb to rant about alleged airline failures on social media. Brenda Song, Jessica Chastain, Porsha Williams, Juvenile and many more stars have also called out their mile-high injustices in the modern public square.

Hope you're not too sore from the flight, Law ... celebs need their stylist in top form!