"Left Outside Alone" is a song by American recording artist Anastacia from her third studio album, Anastacia. Written primarily by Anastacia as well as Dallas Austin, and Glen Ballard detailing the singer's strained relationship with her estranged father. The song was released as the album's lead single on March 15, 2004, to both critical and commercial success. The song peaked at number one in Australia, Austria, Italy, Spain, and Switzerland; number two in Denmark, Germany, Ireland, the Netherlands, and Norway; and number three in the United Kingdom and Hungary, while overall it became the sixth-biggest-selling European single of 2004.

In the United States, "Left Outside Alone" was another success on the Billboard dance charts, where it reached the top position on the Hot Dance Singles Sales, as well as number five on the Hot Dance Club Play. The track received universal acclaim, with praise coming from Anastacia's new musical sound as well as her new singing sound found in the opening of the song being compared to Evanescence's Amy Lee.