The secret to LeBron James' superhuman basketball skills has been revealed -- and it's JUICE ... (the fruits and veggies kind) ... 'cause his wife loves juicing so much she opened up her own shop.

Savannah James tells TMZ Sports, she opened The Juice Spot this Monday in Miami ... and business is already booming.  

LeBron has ZERO financial stake in the place ... but Queen James says she makes her NBA star husband try out all the flavors before they hit the menu.

The MVP's favorite juice is the 20/20 -- which has carrots and coconuts ... and Savannah tells us "I try to stuff the greens in him but it's tough ... he doesn't like those."