Serena Williams' hubby, Reddit co-founder Alexis Ohanian, has traded in his keyboard for free weights and steel chains ... saying he's finally made the life decision to get fit.

But, don't be fooled by the broey gym pic -- he's doing it all for baby Alexis.

"Made a promise to Alexis Olympia Ohanian when I first met her that I was going to start focusing on my health as much as I focus on my business," Ohanian said on social media.

"My legacy is almost starting to talk, so she'll be cheering soon enough."

You can tell he's already made serious gains -- but Mr. "Mediocre at Best" says he's still got a ton of work to do.

No doubt having a workout buddy like Serena helps. Congrats!!