"Lonely" is the second single by South Korean girl group 2NE1 from their 2011 extended play, 2NE1. Later that year, a Japanese version of the song was released and was included on their first Japanese EP Nolza. A short version of the music video was uploaded through 2NE1's official YouTube account.

The song was written and produced by Teddy Park. According to the CEO of YG Entertainment, Yang Hyun Suk, "2NE1’s new song counter feeds the people with an analogue sound unlike the majority who used strong electronics and house music and maybe that is why Will I. Am have [sic] found it interesting". They also released teasers of "Lonely" starting with CL and Minzy on May 9, and Dara and Bom on May 10. The music video was released on May 11, 2011.

By the end of 2011, "Lonely" was downloaded 2,935,930 times, and reached the fourteenth position in the 2011 Gaon Year-End Chart, becoming the third-highest charting of 2NE1's 2011 singles.

The music video for "Lonely" was filmed April 27, 2011, directed by Han Samin, who directed the music video for Big Bang's fourth mini-special album's title song, "Love Song". The video was posted on their YouTube account on May 11, 2011.

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