Justin Bieber -- Training for the Orlando Bloom Rematch

If Orlando Bloom comes around looking to finish what he started, Justin Bieber will be ready.

The diminutive pop star posted a video Thursday afternoon on Instagram ... showcasing his boxing skills while sparring with his bodyguard Hugo.

He captioned the video, "Not a boxer but it's cardio and it's fun so don't judge me lol."

Orlando, you've been warned.