Dean Cain -- ALL ABOARD ... Everyone's Welcome On Cowboys Bandwagon!

Who cares if it's trendy?! Dean Cain says he won't shame celebrities who wanna jump on the Dallas Cowboys bandwagon ... he's welcoming EVERYONE!!

Cain -- who briefly played with the Buffalo Bills -- was fired up in Malibu yesterday after Dallas beat the Seahawks -- and gave a bunch of the credit to head coach Jason Garrett ... who happens to be his good friend.

"Jump on our wagon!" Cain said ... while praising Jamie Foxx and LeBron James for finally breaking out their Cowboys gear this week.

So, will the Cowboys be able to keep up their winning ways? Cain has a theory -- check out the clip.