"It's Always Sunny In Philadelphia" star Charlie Day is living a LIE ... telling TMZ Sports he has no real love for the Philadelphia Eagles ... and actually roots for the New England Patriots.

If you don't watch 'Sunny' (you're missing out, show's hilarious) Day plays Charlie Kelly -- a crazed Eagles fan whose love for the team often gets him into hijinks.

When we got the real-life actor out in L.A. he revealed to us that it's all a goof ... and not only does he not root for the Eagles ... he actually sides with the evilest of all football empires.