W was a Japanese duo formed in 2004 by Up-Front Promotion and associated with Hello! Project. The members consisted of Nozomi Tsuji and Ai Kago, former 4th generation members of the idol girl group Morning Musume. Tsuji and Kago had previously achieved fame through being in the group and the sub-group Mini-Moni.

W's formation was announced at the same time as Tsuji and Kago's planned departure from Morning Musume. Prior to their official departure, they debuted with several cover versions of kayōkyoku songs, including their debut single, "Koi no Vacance" (2004) and their first album, Duo U&U (2004). After leaving Morning Musume, W released their first original song, "Aa Ii na!" (2004), followed by "Robo Kiss" (2004). In 2005, W starred in their first musical, Mysterious Girl Detectives Cara & Mel: The Case of the Stolen Dangerous Violin (2005). They reprised the roles of their characters with the release of "Miss Love Tantei" (2005).

In 2006, W's activities were halted after Kago was suspended for underage smoking. W was scheduled to make a comeback in 2007, but after media reports of Kago dating and smoking again surfaced, her contracted was terminated and Up-Front Promotion announced that W had disbanded.

In 2019, W briefly reunited to make a concert appearance celebrating Hello! Project's 20th anniversary, and their previously cancelled releases, "Dō ni mo Tomaranai" (2019) and W3: Faithful, were released in the same year.