Sam Smith -- End of Rio All-Nighter ... Open McDonald's, I Need Food!! (VIDEO)

Sam Smith got smacked by culture shock -- and probably a hangover -- when his night of partying in Rio de Janeiro did NOT end with a sloppy stop at McDonald's.

This video of the singer -- trying to wrap up an all-nighter -- is hysterical. All Sam wants ... desperately wants, is a freakin' Quarter Pounder with cheese!! Maybe a Big Mac ... depends on how much he drank.

Watch, you can feel Sam's disappointment when a bunch of fans tell him that for some reason ... Mickey D's CLOSED AT 5 AM down there!