Masa Kamaguchi (釜口雅敏, born Masatoshi Kamaguchi) comes from Wakkanai, the northernmost city in Japan, on the tip of Hokkaido. He took up the electric bass at fourteen after hearing records by Jaco Pastorius, and in 1985 he moved to Tokyo to study engineering at Hosei University. In Tokyo he switched to the acoustic bass and took private lessons from Yoshio Ikeda, a former student of Gary Peacock. In 1990 he crossed the Pacific to Boston and enrolled at the Berklee College of Music, where he studied with the drummer Joe Hunt. His own biography lists the Boston players he worked with there as George Garzone, Bert Seager, Hal Crook, Herb Pomeroy and Frank Carlberg, and says he toured Europe and Scandinavia several times.

In 1994 he moved to New York City and stayed for about twelve years. The same biography names Frank Kimbrough, Sonny Simmons, Ben Monder, David Murray, Toots Thielemans, Tony Malaby and Charles Gayle among the musicians he worked with in those years. His first records came from Fresh Sound New Talent, the Barcelona label that documented the New York scene. Lines and Ballads (1999) is a tenor trio with Matt Renzi and the drummer Jimmy Weinstein, cut on 18 February and 28 June 1999, and a second Renzi and Weinstein trio record, Dream Life, followed in Brooklyn on 6 April 2001. Weinstein is the other constant in that decade: he and Kamaguchi are the rhythm section on Frank Carlberg's Prelude (Splasc(h), recorded 30 May 2001) and of NAM, the cooperative with the trumpeter Ahmed Abdullah and the baritone saxophonist Alex Harding. NAM recorded Actual Proof for CIMP in 1999 and played the Vision Festival in New York on 28 May 2001, a set released three years later by Clean Feed as Song of Time.

His New York work ran in two directions at once. In 2002 he recorded Pioneer with the pianist Bert Seager and the drummer Takeaki Toriyama. In February 2004 he spent two days at CIMP's Spirit Room studio in Rossie, New York, with the alto saxophonist John O'Gallagher and the drummer Jay Rosen, and the two Rules of Invisibility volumes that resulted credit all three men as composers and arrangers, with the pieces issued in the order they were played. He recorded Zetrons (2005) as a member of The Cosmosamatics with the alto saxophonist Sonny Simmons. And on 26 April 2005 he was the bassist in Frank Kimbrough's trio with the drummer Paul Motian at Maggie's Farm, the session Palmetto issued in 2006 as Play. Kimbrough, who passed away in December 2020, brought him back for Rumors (Palmetto, 2010, recorded 16 September 2009 with the drummer Jeff Hirshfield).

In 2006 he moved to Barcelona. Fresh Sound's owner Jordi Pujol had been recording him since 1999, and in the summer of 2008 Pujol asked Kamaguchi, the New York pianist Jacob Sacks and the drummer Vinnie Sperrazza to make a record in Barcelona. By Sperrazza's account there were no rehearsals, no sheet music and no second takes; the three called tunes and recorded them, and the result was Barcelona Holiday, issued by Fresh Sound New Talent. The trio has kept going ever since. Beginning with Play Cy Coleman (recorded 14 September 2012), they have made a songbook record about once a year, mostly at Michael Brorby's Acoustic Recording studio in Brooklyn: Johnny Mandel, Tadd Dameron, Lee Morgan, Benny Golson, Sonny Rollins, then John Coltrane and Harold Arlen on a single day in January 2020. Sperrazza wrote in September 2024 that the trio was preparing its tenth album, built on the music of Elmo Hope; Play Elmo Hope was recorded on 20 September 2024 and released digitally in October 2025.

Spain also made him a regular on the local scene. Background Music (Fresh Sound New Talent), a trio record with the American tenor saxophonist Noah Preminger and the drummer Rob Garcia, was recorded at Estudi Laietana in Barcelona on 7 December 2010 and released in 2014. He appears on albums by Lluis Vidal, Albert Sanz, Ivo Sans, Álvaro Torres, Sergi Felipe and others, made a duo album with the saxophonist Javier Vercher, and in 2014 released Solo Masa, a solo double bass set recorded live on 10 May 2013 that runs from a fifteen-minute opener to "Prelude to a Kiss" and "Donna Lee". His 2014 credits also include Sounds From the Harbor, a quartet billing with the saxophonist Andy Sheppard, the guitarist Marco Tindiglia and the percussionist Michele Rabbia.

The record that put him back alongside Japanese colleagues is Boundary (ESP-Disk', 2018), with the pianist Megumi Yonezawa, who is also from Hokkaido, and the Tokyo-born drummer Ken Kobayashi. Recorded on 4 June 2017 at Park West Studios in Brooklyn and produced by Steve Holtje, it has liner notes by Matthew Shipp and ten tracks, including one standard, "I'll Be Seeing You", with the rest collectively improvised. The session was booked while Kamaguchi was visiting New York from Spain; the label's biography of the trio, from 2018, says he lives in Madrid and visits New York for a month twice a year. In 2020 he joined the trombonist Samuel Blaser, the guitarist Marc Ducret and the drummer Gerry Hemingway on Moods, recorded live at Moods Jazz Club in Zurich on 30 October 2020. Blaser's Rosina, with Russ Lossing, Kamaguchi and Billy Mintz, was released on 15 October 2026.

Kamaguchi works in two idioms and moves between them freely. One is the post-bop piano trio, the setting of Barcelona Holiday, Pioneer and the Fresh Sound songbook albums, where the repertoire is a single composer's book, from Cy Coleman and Johnny Mandel to Tadd Dameron, Lee Morgan, Sonny Rollins and Elmo Hope. The other is free improvisation, heard on Boundary, the two Rules of Invisibility volumes, Zetrons and NAM's Song of Time. The Global Music Foundation's biography of him describes an accurate and intuitive player with a warm, rich tone and a modern style, open to a wide span of influences. A reviewer of Boundary called the trio's music a loosely structured dialogue between the three players.

His starting point was electric: Jaco Pastorius records at fourteen. His acoustic training came in Tokyo from Yoshio Ikeda, who had studied with Gary Peacock, and then at Berklee with Joe Hunt. In the trio with Vinnie Sperrazza and Jacob Sacks there is no horn, so the bass carries harmonic and rhythmic information both, and Play Cy Coleman and Play Sonny Rollins are good places to hear that. Solo Masa removes every other voice and runs one bass through "Donna Lee" and "Prelude to a Kiss" alongside long improvised pieces.

Japan is the beginning of his story rather than its setting. He was born in Hokkaido and trained in Tokyo from 1985 to 1990, and he has recorded with the Japanese-born musicians Megumi Yonezawa and Ken Kobayashi, but the bulk of his catalog was made in New York and Spain with American and European players.