LeBron James and the Cavs went big the night before Game 1 of the NBA Finals -- teaming up for a big dinner at a famous S.F. steakhouse ... TMZ Sports has learned. 

The team hit up EPIC Steak -- the same place SF Giants star Hunter Pence had his crazy "Game of Thrones" themed wedding last year. 

James and the rest of the team ate like kings -- feasting on steak, shrimp, meatballs, filet mignon and wild striped bass. 

We're told some of the players had wine to compliment the meal -- but don't freak out, no one got even remotely close to being lit. They left around 11:30. 

After dinner, the guys were in a great mood -- dancing and singing as a team -- before getting back on the team bus.

No word on what the Warriors had for dinner ...