The two-time Oscar nominee, who is in London to promote his new film Men In Black 3, shot some hoops, jumped the hurdles and threw punches with Olympic hopefuls Drew Sullivan, captain for the GB basketball team, hurdler Perri Shakes-Drayton, super heavyweight boxer Anthony Joshua and triple jumper Yamile Aldama.
"I've always loved sports - I love basketball and I love boxing. But I've never considered myself an athlete," Smith said.
The 43-year-old previously dabbled in boxing for his Oscar-nominated performance as Muhammad Ali in 2001's Ali, and has a share in the Philadelphia 76ers basketball team.
"I was a little bit goofy until I was about 14 or 15. The first time I ever saw myself look good athletically was in the first Bad Boys," he continued.
"Michael Bay shot me running in slow-motion and I thought, 'Hey, that doesn't look half bad' but other than that, I've never been really athletic."
Smith - who will attend the gala premiere of his new film with co-stars Josh Brolin and Emma Thompson tonight - admitted to enjoying the exercise, which took place at Ethos Sports Centre in South Kensington.
"That is fun. That's the first time I've ever done the hurdles and the triple jump, and had any interaction with it, so I'm gonna go ice myself down," he said.
"That stuff is a lot harder than it looks on TV. I'm clearly not a hurdler. That gives you an idea of the peak physical condition that athletes have to be in. I had a little taste of it when I made Ali but there's a whole other level of conditioning. I'll be at the Olympics to watch," he added.
As for the boxing, Smith joked: "I was 30 when I made Ali. Now I'm 43."