South Park’s longest season by episode count is Season 2.
Season 2 aired from April 1, 1998, through January 20, 1999, and contains 18 episodes. That total remains the series record, even as later seasons continued experimenting with serialization, shorter runs, and event-style specials.
The season opened with “Terrance and Phillip in Not Without My Anus,” an April Fools’ episode that frustrated viewers expecting the resolution of Season 1’s Cartman-father cliffhanger. Comedy Central moved “Cartman’s Mom Is Still a Dirty Slut” forward in response to the reaction, helping make the season’s broadcast history unusually complicated.
A common mix-up is confusing the longest season with the most recent or most heavily serialized one. Season 2 is the clear answer because its 18-episode total exceeds the counts of the surrounding seasons, including Season 1’s 13 episodes and Season 3’s 17.