When Michael J. Fox appears tonight on Jimmy Kimmel Live—just another way to say "happy Back to the Future day!"—he will be wearing a pair of Nikes that he wore in the movies. No, not those. You were likely thinking of the Nike MAG, the super-high, power-laced futuristic models he was given by Doc Brown (Christopher Lloyd) when he arrived in 2015. Those were just the sneakers he changed into. He'll be wearing the ones he changed out of when he arrived in Hill Valley from 1985 back to 1955 (as seen in the below scene).
As legend has it, Fox wore red-on-white Nike Bruins to the set of Back to the Future, a pair that was originally gifted to him by Nike. Seeing that the costume designer had forgotten to bring sneakers for Marty McFly's character, director Robert Zemeckis—declaring the Bruins to be "exactly what a teenager would wear"—allowed Fox to wear his own kicks, but not before removing the tongue label and NIKE block lettering from the heel tabs. His battered Bruins are clearly visible throughout, especially in the skateboarding scenes.
Did this little wardrobe mishap in 1985 help lead to the iconic Nike MAG in the 1989 sequel that got some major closeups during the hoverboard scene (below)? That we don't know for sure. But McFly's Bruins—which did re-release today, complete with unbranded tongue and heel—deserve their own spot in Back to the Future lore.

Read more of Collider's coverage of "Back to the Future Day"