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 .