Ken Watanabe (渡辺建) is a Tokyo-born bassist who co-founded the fusion band Prism in 1975 and played in it for more than twenty-five years. He wrote some of its best-known pieces, among them "Slow Move", "Unforgettable", "The First Sky and the Last Sea" and "Come On", and he sang and worked bass pedals while playing the instrument. Outside Prism he became a first-call session player on Tokyo's fusion and studio scene, and later a fixture of musical-theatre and orchestral pit bands. He is not the film actor of the same name (渡辺謙).

He was born on 28 June 1953 in Suginami, Tokyo. At university he was preparing for a career as a simultaneous interpreter in English, and his first band, Hi-Jacks, played mostly at American military camps; that is where he started as a bass player. In 1974 he enrolled in the bass department of the Yamaha Nemu Music Academy, where he studied double bass with Yasuo Arakawa and electric bass with Masahiko Rokugawa.

In 1975 he and the guitarist Akira Wada formed Prism. The band signed to Polydor in 1976 and released its self-titled debut album in 1977, whose first pressing sold out within hours, according to his official site. Shortly afterwards the band supported Eric Clapton at the Budokan in Tokyo. The second album, Second Thoughts/Second Move (1978), carries three of his compositions, "Shake Your Head", "Bang Your Head" and the ballad "Slow Move". On Prism III (1979) he wrote and arranged "Living For The Daylight", "My Little Trick" and "Memories Of You~When You Were Gone", and the 1979 live double album Prism Live opens with that last piece, which begins with a bass solo. Surprise (1980) was recorded in four days and includes his "Upside Down", "Thoughts" and "Unforgettable". Visions (1982), made in Hollywood, includes "The First Sky and the Last Sea", a piece he kept playing for decades and re-recorded on his 2022 solo album.

From the mid-1980s Prism worked as a guitar-bass-drums core with Wada, Watanabe and the drummer Mansaku Kimura, adding keyboardists including Jun Fukamachi on Nothin' Unusual (1985), Dreamin' (1986) and The Silence of the Motion (1987). "Take Off", which Watanabe co-wrote with Wada, was used in a Mild Seven Menthol television commercial, and his "Si" was used in a California walnuts advertisement. "Come On", from the 1987 album, became the opening theme of a TV Asahi news programme. In 1990 to 1992 the band made an environmental trilogy, Mother Earth, Rejuvenation and A Personal Change; in concert he played "Awakenin'" from the first of them while singing and playing bass pedals at the same time. Whiter (1997) was his last studio album with the band, and he left Prism in 2001. He returned as a guest bassist for the band's Homecoming concert at Club Citta on 31 July 2004, released on DVD.

Alongside Prism he was in demand in the studios. He played bass on Toshiyuki Honda's Easy Breathing (1980), and he was a member of the rhythm section on Naoya Matsuoka & Wesing's Latin-fusion album Danzon (1981), then a guest on their The Show (1981) and bassist on The September Wind (1982) and Matsuoka's How Romantic! (1986). He played in Kazumi Watanabe's Mobo band on Mobo Club (1984) and Mobo Live (1985), and on Akira Wada's solo records The Guitar (1981), Yellow Moon (1984) and Love Islands (1984). His credits run on into pop, kayokyoku and rock: Hitomi "Penny" Tohyama, Saki Kubota, Yoshiko Tanaka, Seri Ishikawa, Yusaku Matsuda and Kenichi Hagiwara, as well as the drummer Munetaka Higuchi's hard-rock album Destruction (1983), on which he also wrote and arranged "Law Breaker:M". Much later he played bass on the drummer Hiroyuki Noritake's Dreams Can Go (2000).

Teaching has been a steady thread. In 1983 he and the guitarist Katsumi Kobayashi made Guitar Shape-Up for Victor, and he put out his own bass album, Bass Shape-up, the same year; it was reissued on CD in 2017 with the Guitar Shape-Up tracks added. A scale-study video for bassists followed in 2004, and from 2014 he gave private lessons.

After leaving Prism he worked as a support musician and in stage productions. He played in the musical Blood Brothers in its first Japanese run and its revival, and again when it returned to Theatre Creo in Ginza in 2009, in Zorro the Musical, and in a Fame show for the Musical Academy. From 2004 to 2015 he was the bassist on the Disney on Classic orchestral tours with orchestras including the Tokyo Philharmonic, and in May 2008 he played in the jazz combo that sat on a balcony of the New National Theatre in Tokyo for Zimmermann's opera Die Soldaten. He began solo performances for bass and voice at the Daphne jazz club on 3 February 2007, and in July 2011 he formally joined Yuzo Gucchi's band the Gucchiiz after supporting it as bassist. His solo album I've Been There appeared in June 2022 on his own KWM label, with new versions of his Prism-era pieces "The First Sky and the Last Sea" and "Black Wings" and a multi-movement "Dreamin' Suite".

Watanabe plays both fretted and fretless Yamaha basses, and his bass solos are a feature of Prism's live records. Prism's own sound combined a hard-rock foundation with jazz, ethnic and progressive-rock elements, and its early records drew on Santana, Return to Forever, Al Di Meola and Jeff Beck. His compositions for it range from "Shake Your Head" to the ballad "Slow Move", the long-form "The First Sky and the Last Sea" and the baroque-leaning "Blue, Green&Red" on Rejuvenation. His gear is mostly Yamaha: BB-2000, BB-3000 and BB-5000 basses and later six-string TRB models, with a Moog Taurus bass-pedal unit on Live Alive (Absolutely) (1982).

The rock side of his taste is on the sleeve of Nothin' Unusual (1985), where three of his pieces are dedicated to Captain Beyond, King Crimson and Pink Floyd; he has also written on his site of admiring Pink Floyd, Sting, Don Henley and Bonnie Raitt. His work is best filed under fusion and jazz-rock. A large part of his career, in pop sessions and in musical-theatre and orchestral pits, sits outside jazz, and his Latin-jazz credits come from the Wesing band.