"Office" star Ed Helms is officially boycotting Chick-fil-A ... claiming the restaurant chain's anti-gay stance has caused him to lose his appetite.

Helms is responding to a statement issued by Chick-fil-A president Dan Cathy ... who recently told a Christian publication that when it comes to accusations his company is actively opposed to gay marriage, Chick-fil-A is "guilty as charged."

Cathy claims his chicken people support the "biblical definition of the family unit." In fact, Chick-fil-A reportedly donated nearly $2 million towards religious groups that oppose gay marriage in 2010 alone.

The comments clearly didn't sit well with Helms ... who tweeted, "Chick-Fil-A doesn't like gay people? So lame. Hate to think what they do to the gay chickens! Lost a loyal fan."