Showtime Declares War On Mayweather vs. McGregor Pirates

They thought they were slick ... dozens of websites aiming to livestream the Mayweather vs. McGregor fight for free.

But Showtime is preemptively striking the potential pirates with a big, fat lawsuit ... mateys.

The network is ripping a page from the Mayweather vs. Pacquiao playbook -- where they scouted out a bunch of potential illegal streaming sites and hit 'em with a legal blow before the fight.

There's obviously a TON of cash on the line -- some say the fight could bring in more than $500 MILLION ... so, they ain't lookin to let anyone skim from their pockets.

In the suit, filed in federal court, Showtime is asking a judge to issue an injunction to block the sites from streaming the…