John Kaizan Neptune (ジョン・海山・ネプチューン) is an American shakuhachi player and maker who first came to Japan in 1973 and has built his entire career there. He was born in Oakland, California, on 13 November 1951, played trumpet in high school and later drummed in a rock band, and grew up surfing. At the University of Hawaii he studied ethnomusicology; he has said he wanted to learn the tabla, found no teacher, and took up the shakuhachi as his second choice. In 1971 he began lessons in Hawaii with a Japanese Buddhist priest.

In 1973 he spent a year in Kyoto, went back to Honolulu to finish his degree, and then returned to Kyoto for several years of intensive study in the Tozan school, under the shakuhachi master Genzan Miyoshi (三好芫山). In March 1977 he received the shi-han, the master's certificate, and the professional name Kaizan (海山), "Sea Mountain." He has lived in Japan ever since, and for decades in Kamogawa, a surfing town on the Chiba coast. His earliest Japanese recording credits are shakuhachi parts on two Teichiku LPs by the Oriental Mechanic Band, in 1977 and 1978. As a non-Japanese player of a Japanese instrument he drew considerable public interest from the start.

His first jazz session on record is a small-group date. On 20 September 1978 the pianist Norio Maeda recorded We Three Jazz for the Adam label as a PCM digital recording, with Yasuo Arakawa on bass and Takeshi Inomata on drums, and Neptune is credited on shakuhachi, with a track called "Bamboo Blues" carrying his name. His debut as a leader, Bamboo Textures, followed on Toshiba in 1979, then Joy on Kosei in 1980, and in 1980 the record that made his reputation: Bamboo, made with the Arakawa Band, a fusion group led by the saxophonist Tatsuhiko Arakawa. It was recorded at Toshiba EMI Studio in Tokyo between April and June 1980 and arranged by Arakawa and Osamu Shiomura, with liner notes that included a contribution from the American critic Leonard Feather. Bamboo received an Excellence Award at Japan's Agency for Cultural Affairs Art Festival (文化庁芸術祭優秀賞), and the reports repeated on his Wikipedia entry and on the album's reissue pages say it was the first time the prize went to a jazz recording and the first time it went to a foreign artist. Inner City Records issued it in the United States.

The next three years were his most jazz-facing. Shogun (Express, 1981), billed to John Kaizan Neptune and Mu'ryō, was recorded in October and November 1980 in Tokyo with Takao Naoi arranging and playing guitar and a Tokyo horn section, and Discogs tags it jazz-funk. Mixed Bag (Eastworld, 1981) was recorded and mixed in Glendale, California, at Monterey Sound Studios, with Don Grusin arranging and co-producing; the single "Soft Melody" / "Shakuhachi Dance," taken from it, lists Grusin as co-producer. For West of Somewhere (Eastworld, 1981) he went to New York, where the album was recorded at Blank Tape Studio on 12 to 21 August 1981 with Buster Williams on bass, Idris Muhammad and Alex Acuña on drums, Cornell Dupree and Akira Kon on guitars, Kenny Kirkland on keyboards, Becky Friend on flute and Yvonne Lewis on vocals. It includes his arrangement of Stevie Wonder's "Isn't She Lovely." In 1982 he made Two In Flight with Naoi, recorded from 28 June to 9 July with Takeru Muraoka on tenor saxophone, Souichi Noriki on synthesizer and Nobuo Kurata on piano, and Digital 45 (DAM, 1982), a 45 rpm LP of shakuhachi with two basses, including Kunimitsu Inaba, and percussion, which Discogs tags free improvisation and contemporary jazz and which includes a reading of "Hey Jude."

From the mid-1980s the records widen. The Circle (Denon, 1985) set the shakuhachi against piano, bass, drums, koto, Indian violin and Indian percussion, Jazzen (Denon, 1987) against guitar, bass, vibraphone and percussion, and a Jazzen track, "Bamboo Born," which Neptune produced himself, was chosen for Denon Jazz Sampler Volume 2 (1987), a label compilation that sat his piece beside tracks from Peter Erskine and Eliane Elias albums. Tokyosphere (JVC, 1988) and Sky Spirit (JVC, 1991) followed. In September 1991 he recorded the unaccompanied Words Can't Go There at his own dome house in Kamogawa. Kite (Kosei, 1992) was recorded in a church in Germany with the Modern String Quartet, River Rhythm (Kosei, 1994) in Madras, India, in February 1993 with South Indian musicians, and Prime Numbers (1994) with the koto player Yasuko Watanabe, issued on his own Nep Tunes label with Victor. He plays shakuhachi and percussion on the taiko drummer Kenny Endo's Eternal Energy (1994), and he is the guiding figure behind the bamboo-percussion group TakéDaké, which plays his compositions on instruments many of which he built; its album Asian Roots (Denon, 1998) is billed to TakéDaké with Neptune.

He returned to the jazz repertoire with Shakuhachi Mellow Jazz (Nep Tunes, 1999), fifteen standards from "In a Sentimental Mood" to "Over the Rainbow" with guitarist Takao Naoi, bassist Toru Kase and drummer Mark DeRose, and made Bamboo Magic (Nep Tunes, 2007) with Indian and Japanese players. His son David Neptune directed Words Can't Go There, a documentary about his father's path from California surfer to shakuhachi professional, which premiered in Japan on 5 October 2019. Neptune still lives in Kamogawa, where he grows bamboo and makes his own shakuhachi and other bamboo instruments, and he turned 74 in November 2025. A remastered vinyl reissue of Bamboo on Universal Music Japan (UPJY-9545) is listed by retailers for 2026.

Neptune is a borderline case for a jazz catalogue, and the plain description is that he is a Tozan-school shakuhachi master who has spent half a century working in jazz, fusion and cross-cultural settings. The jazz case is real: Bamboo with a fusion band led by a jazz saxophonist, the arts-festival honour reported as a first for a jazz recording, a first jazz credit on a 1978 Norio Maeda trio date, a 1981 run of records made with American players such as Buster Williams, Idris Muhammad, Cornell Dupree and Don Grusin, Denon's decision to place his track on a jazz sampler, a 1999 album of standards, and Discogs tags that list jazz on most of his leader and co-leader releases. Wikipedia's infobox lists jazz and jazz fusion ahead of new age and world music, and Wikidata gives jazz as his genre. The case against is also real: much of his later output is filed under folk, world or new age, Discogs files Kite as classical, Words Can't Go There and Prime Numbers are traditional or chamber-scale shakuhachi music, and the TakéDaké albums are world-music projects.

The sound at the jazz end of his work is a bamboo flute with a breathy, sustained attack, played over a rhythm section rather than in place of one. On Bamboo and Shogun the shakuhachi sits on top of jazz-funk and fusion arrangements and horn sections; on West of Somewhere it carries a Stevie Wonder melody over New York session players; on Digital 45 it plays over two basses with almost nothing else around it. He composes and arranges much of his own material and produces many of the records himself. At the other end he is a certified traditional player, with a track called "Honshirabe" on Words Can't Go There and a catalogue of koto and shakuhachi chamber pieces including Five and Thirteen Are Prime Numbers.