Ariana Grande -- I Donut Always Stick to My Diet (VIDEO)

Ariana Grande cheats -- like most of us do -- on her diet, and she caved to the pressure in that donut shop.

Ariana reminded us last week, she's vegan ... but check out the video. As the singer and her BF Ricky Alvarez were leaving Wolfee Donuts in Elsinore, CA ... she got verrry curious about some of the delicious pastries surrounding her.

There are such things as vegan donuts, but we checked ... Wolfee doesn't carry 'em. The best part is she pulled the ole, "I want a half of one" trick.