LeBron & Jamie Foxx -- Dust Off Dallas Cowboys Gear ... All Aboard the Celebrity Bandwagon!

It's trendy to be a Dallas Cowboys fan again ... just ask LeBron James and Jamie Foxx.

After the 'Boys beat up on the World Champion Seahawks in Seattle last night ... the closet opened and the celebrity bandwagoners began spilling out.

LeBron and Jamie have admitted to being Cowboys fans in the past -- but hadn't posted a single picture on Instagram supporting the team ... until now.

Funny how that works.

Also, LeBron touts himself as a Browns fan as well ... funny, he wasn't wearing any Cleveland gear after THEIR big win over the Steelers yesterday.

One guy who HAS been down with Big D all season long -- former "Dallas" star Josh Henderson ... who DEFENDED Tony Romo even when everyone else was calling for his head.