Exclusive Details

Conor McGregor's movie career is officially ON -- 'cause we're told the UFC's 13 second knockout king has agreed to a role in the new 'xXx' movie.

The Irish superstar has said that Hollywood has been knocking on his door for a while -- and Vin Diesel is a huge fan when it comes to putting fighters in his flicks (like Ronda Rousey in "Furious 7").

In fact, Diesel posted on Instagram Monday suggesting Conor was on board for "xXx: The Return of Xander Cage."

Now, our sources connected to the situation tell us ... Conor HAS verbally agreed to play a role in the flick -- though it's unclear how big of a part it will be.

Of course, things happen in Hollywood and Conor could always pull out -- but for now, we're told it's a go.

As we previously reported, Mark Wahlberg is also interested in the Notorious one for an upcoming project.