The first season of the American comedy television series It's Always Sunny in Philadelphia premiered on FX on August 4, 2005. The season contains 7 episodes and concluded airing on September 15, 2005.

This season, the gang inadvertently turns Paddy's Pub into a gay bar after one of Sweet Dee's acting class members invites his gay friends over to drink while Mac and Charlie try to prove to Charlie's crush that they are not prejudiced against African Americans. Then Charlie becomes a father when an old flame reveals that the son she has is his—and Charlie attempts to get a paternity test at a free clinic but the old flame finally reveals that is not true, while Mac and Dennis frequent pro-choice and anti-abortion rallies to pick up women. The gang then turns their bar into a safe haven for underage drinkers—and find themselves regressing to their high school ways when they get invited to the prom. Charlie fakes a cancer diagnosis and the gang looks for a woman to sleep with him before he dies.

The gang then gets gun-crazy when their bar has been robbed and Dee dates a man who may be the culprit. Following that, the gang finds a dead man in one of their booths and Dee and Dennis visit their ailing (Nazi) grandfather at a nursing home. Finally, Charlie is suspected of being a victim of child molestation when a former gym teacher is accused of sexually molesting his students—and Mac is livid that he was not one of the victims.

The first season received positive reviews. On Rotten Tomatoes, it has an approval rating of 78% with an average score of 7.9 out of 10 based on 18 reviews. The website's critical consensus reads, "It's Always Sunny in Philadelphia's coarse humor and shabby style may be rough around the edges, but this sinful sitcom wrings a surprising amount charm from its band of charmless underachievers."^[4]