Michelle Williams ain't going to "Cater 2 U" if you're taking your shoes off on an airplane ... taking to social media to call out her seatmate who went barefoot on a flight.

The singer-songwriter hopped on Instagram to express her displeasure with the person sitting next to her ... sharing a snap of their toes -- totally uncovered and poking out from behind the seat barrier.

Michelle wrote, "I PROMISE YOU BETTA GETCHO FOOT FROM ME!!!!