"Purple Line" is Tohoshinki's 16th Japanese single, written by the Korean composer who made Tohoshinki's biggest hits in Korea such as "Rising Sun" and ""O"-Jung.Ban.Hap." It became Tohoshinki's first Japanese single to reach #1 on the Oricon Weekly sales chart in Japan, making the group the first foreign male group or boyband to top the Japanese charts and second Korean artist after BoA to do so. "Purple Line" was Tohoshinki's first single that was originally released in Japanese and later released in Korean as a single. According to the members, the name "Purple Line" refers to the thin rays of light on horizon when the sun sets. Despite being the first #1 single from Tohoshinki, it is not their strongest single in terms of physical sales, with "Break Out!" having the biggest lead in Japan. It was released a week before their third Japanese album T which included the song in its tracks.
The music video was shot in South Korea, the first time for a Japanese single by the group. The producers used new strategies to market the song and the video. First, a shortened music video was released in Japan for the Japanese version of the song. On January 24, 2008, a Korean version of "Purple Line" was also released. On February 4, 2008, SM Entertainment surprised fans with the release of the full music video; however, with the Korean version of the song. A Japanese version of the full music video was also released a few days later.