"Who Knew" is a pop rock song written by Pink, Max Martin and Dr. Luke for Pink's fourth album, I'm Not Dead (2006). It was released as the album's second single worldwide in 2006, and was re-released to U.S. radio as the album's fourth single in the United States in 2007, after the success of "U + Ur Hand".

Pink has stated that "Who Knew" is her favorite song from her first compilation, Greatest Hits... So Far!!!, as it can be about almost everyone. Lyrically, it is about losing a friend because of drugs. The music video shows Pink singing in a county fair, with a storyline about two young lovers who end up being separated after the guy begins using drugs. The song is Pink's longest chart runner on the Hot 100, spending 36 weeks before dropping out. (Note that "Who Knew" has various Official single covers)

In an interview for the Daily Mail on 26 May 2006, Pink said this song is about losing a loved one to drugs: "My life was insane, I was [in her youth] out of control and doing lots of stupid things. Some of my friends were selling crack, and I got into drugs too. I've always been honest about that, although I don't like going into details. I've got lots of young fans and I wouldn't want to give them any ideas. I found a friend dead from a drug overdose when I was 14. He was a male friend, not a boyfriend. Most of the people at the funeral were just children. It should have been a wake-up call for me, but it wasn't." Billboard magazine, in its second review of the song (to accompany the U.S. re-release), called it "another cool, smart, plenty passionate melodic jewel about the death of a friend and/or friendship, with an instantly identifiable, quick-to-the-point, singalong track—maintaining Pink's stance as the smartest female millennium-era singer/songwriter, dazzling in star quality as alterna-pop's premier princess."

Alecia Beth Moore (born September 8, 1979), better known by her stage name Pink (often stylized as P!nk), is an American singer-songwriter, musician and actress. After her three-year career with the contemporary R&B girl group Choice, in 2000 she released her first single "There You Go," from her debut album Can't Take Me Home. The song garnered commercial success, peaking at #7 on the Billboard Hot 100. In 2001, she released her second, more pop rock-oriented, studio album, Missundaztood. The album went on to become a critical and commercial success, with estimated sales of 13 million. The album produced four singles, "Get the Party Started," "Don't Let Me Get Me," "Just Like a Pill" and "Family Portrait," each entering the Top 20, with "Get the Party Started" being her highest charting solo single (tied with "Most Girls") until "So What" in 2008, peaking at #1 on the Billboard Hot 100.