"Alone Again (Naturally)" is a song by Irish singer–songwriter Gilbert O'Sullivan. It was released in 1972, and in total spent six weeks, non-consecutively, at #1 on the United States Billboard Hot 100 singles chart. In Casey Kasem's American 'Top 40 of the 1970s', "Alone Again (Naturally)" was #5 (Debby Boone's "You Light Up My Life" was #1). The track reached #3 in the UK Singles Chart.

It is an introspective ballad, starting with the singer telling of his plans to commit suicide after being left at the altar, and then telling about the death of his parents. O'Sullivan has said that the song is not autobiographical, as he did not know his father (who died when O'Sullivan was 11) very well, and that his father had mistreated his mother. "Alone Again (Naturally)" is included on O'Sullivan's The Berry Vest of Gilbert O'Sullivanalbum (2004) on the EMI record label. Big Jim Sullivan plays the guitar break in the original recorded version of the song.

The landmark 1991 copyright case Grand Upright Music, Ltd. v. Warner Bros. Records Inc.centered on the unauthorized use of a sample from "Alone Again (Naturally)" by rapper Biz Markie.

The song has been featured in a handful of movies and TV shows, including Summer Time Machine Blues, Stuart Little 2, The Virgin Suicidesand Stuck on You. It was also the opening theme song of episode 24 of the Japanese anime Maison Ikkoku(Sullivan's later hit "Get Down" was the closing theme of that same episode, but the producers were unable to use the songs for more than one episode due to copyright problems).

In 1972, the Dutch comedy duo Van Kooten en De Bie wrote a Dutch version of the song, called 1948. It deals about nostalgic feelings of a sober, but happy childhood in the year 1948, just three years after World War II.

Dame Shirley Bassey covered the song on her 1976 album Love, Life, and Feelings.

In 2004, Ehud Manor wrote a Hebrew version of the song. This version is called "VeShuv Levad" (Hebrew: "ושוב לבד"‎, "Alone Again"), and sung by Tiki Dayan, an Israeli actress. The Hebrew version was used as the theme song for the Israeli drama series Matay Nitnashek, in 2007.

In the season 17 episode of The Simpsons, "The Wettest Stories Ever Told", Comic Book Guy uses it as inspirational music. During the second series of the British television drama Life on Mars, it is used as an end theme of one episode to signify the despair of the lead character, Sam Tyler.

In May 2000, a cover of the song appeared on Diet for a New Americaby 58 - a side project by Motley Crue's Nikki Sixx. The song was covered in 2005 by Elton John and the Pet Shop Boys for a rare 2005 promo of Gilbert O'Sullivan. This cover version was never released commercially.

Cass Elliot also covered song live.

The song was also used in Ice Age: Dawn of the Dinosaurs, the lyrics were changed to be more humorous and romantic.