Christopher 'Big Black' Boykin, who was the other half of Rob Dyrdek's hit MTV show "Rob & Big" -- has died ... TMZ has learned.

His rep tells us Chris died Tuesday morning. No official cause of death yet, but multiple people connected to Chris tell us they believe it was a heart attack.

He was Rob's best friend and bodyguard on their reality show, which ran from 2006 to 2008 -- and also later appeared on 'Fantasy Factory.'

The pals got into a bunch of random adventures -- like breaking Guinness World Records, exorcising their house, installing an ATM ... and, of course, catching people with their net gun.

Chris and Rob's relationship became strained while shooting their show. Rob talked about their falling out a couple of years ago.

They eventually patched things up. Not only did they work together on 'Fantasy Factory' ... but Chris recently posted a throwback pic of them from the show's heyday.

Before he was a TV star, Boykin served in the U.S. Navy. He's survived by his 9-year-old daughter.

'Big Black' was 45.

#RIP