Marty McFly Shoe Buyer -- "Spur of the Moment Decision"

The British rapper who dropped $37,500 on the badass new Marty McFly kicks tells TMZ, he wasn't planning to buy them at last night's L.A. auction -- he just got caught up in the moment.

Tinie Tempah tells us, "It was a total spur of the moment decision. I was really touched in the moment by the presentation and next thing I knew I owned these iconic sneakers."

Tinie -- a longtime sneaker-collector -- says he's also a HUGE fan of "Back to the Future" ... so the shoes "were an all-around great purchase that will go on to help [The Michael J. Fox Foundation]."