"Ain't It Funny" is a song recorded by American entertainer Jennifer Lopez. It was written by Lopez and Cory Rooney for the Adam Shankman-directed romantic comedy The Wedding Planner (2001). Shankman, however, felt that the song had too much of a Latin-influence to be featured in the film, and it was instead included on Lopez's second studio album, J.Lo (2001). "Ain't It Funny" was released as the third single from the album on June 20, 2001, by Epic Records.

While watching a rough cut of The Wedding Planner (2001) in Canada, Jennifer Lopez and Cory Rooney brainstormed ideas for its accompanying soundtrack. The two "pulled different thoughts and feelings" from the film's scenes. The scene where Lopez's character is in the backseat of a car listening to Matthew McConaughey and Bridgette Wilson's characters talk about the wedding made Rooney think of the line "ain't it funny how some feelings you just can't deny and you can't move on even though you try". According to Rooney, "all those words and melodies just started coming" to him while watching the scene. After he conceived the idea for "Ain't It Funny", Rooney returned to his home and "roughed out the track" and came up with the song's chorus. He then worked with Lopez for an hour to complete the song.Tommy Mottola was so enthusiastic about the song that he contributed to its melody.Adam Shankman, however, felt that "Ain't It Funny" had too much of a Latin influence for the film, due to its usage of timbales. Lopez disagreed with Shankman and the two later agreed to use "Love Don't Cost a Thing" in the film.

"Ain't It Funny" is a Latin song, with a duration of four minutes and six seconds (4:06). The song contains a Mecca disco beat and a "bullfight trumpet", as well as a "solid dancefloor thump". Lyrically, Lopez sings about "creating the perfect romance in your mind, then facing reality when Mr. Right is less than ideal".

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