Marshawn Lynch might be the BEST MCDONALD'S CUSTOMER EVER ... 'cause he gave $500 to a Mickey D's staffer last week ... because he wanted to help the kid out.
It all went down at McDonald's in Dallas last Sunday after the Seahawks beat the Cowboys -- when Beast Mode walked into the joint and struck up a conversation with 17-year-old Terrance Downs, who was mopping at the time.
"I told him I liked his shoes -- navy blue Buscemis -- and how I wanted to buy a pair and we all started talking," Downs tells TMZ Sports.
Lynch asked him about school and what he wanted to do in life, and Downs told him he's into fashion and wants to own a boutique one day.
That's when Lynch whipped out $500 and handed it over ... telling Downs, "If you're serious about getting those shoes, here's some money to help you get 'em. My job is to continue to see you grow."
A rep for the owner of the McDonald's confirmed the story -- adding, "Marshawn left a huge positive impact on Terrance."
As for Downs, he tells us he went out and bought the shoes ... and says he's even more motivated to become successful so, "I can do the same thing for someone else one day."