Koji Yamashita (山下弘治, Yamashita Kōji) was born in Nagoya on October 9, 1967. He studied earth sciences at Nagoya University's Faculty of Science and began playing the double bass in local live houses while still a student, so the music was already a working life before the degree was finished. After graduating he joined the trio and sextet of the pianist Yutaka Shiina. In December 1993 he was one of two bassists on a session in New York by The Jazz Networks, the band of Japanese and American players that Shiina led, at Clinton Recording Studios. The record was Blues 'N Ballads (Novus J, BVCJ-616), taped on December 14 and 16, released in Japan on April 21, 1994, with the trumpeter Roy Hargrove, the tenor saxophonist Joshua Redman, the alto saxophonist Jesse Davis, Shiina on piano, Masahiko Osaka on drums and the American bassist Rodney Whitaker alongside him. Discogs credits Yamashita on tracks 1 to 7 of its nine, and Roy Hargrove's official site lists both bassists. His teaching biography counts it as the group's third album.

He moved to Tokyo in 1994 and joined the group of the trombonist Shigeharu Mukai, a fellow Nagoya musician he had known before the move, and he dates his career as a full-time jazz bassist from that year. The Jazz Networks kept recording with him: The Other Day (Novus J, BVCJ-637), released September 21, 1996, has Hargrove again, the alto saxophonist Antonio Hart and the tenor saxophonist David Sanchez in a front line of four, and credits three bassists, Yamashita among them. Discogs does not say which of the three plays on which of its nine standards.

Around those albums his name starts turning up on Japanese sessions of every size. In 1996 he played the first six tracks of For Musicians Only (Paddle Wheel, KICJ 274), the trumpeter Tomonao Hara's side of a split record with the trombonist Eijiro Nakagawa, in a quartet with Shiina on piano. The Makoto Terashita Trio's Ihatov was recorded on August 11, 1996 and issued on vinyl the next year, with Masahiro Tajika on drums. On June 9, 1998 he sat down for Jazz Shinsen-Gumi (Paddle Wheel, KICJ 354), a twenty-one-track session produced and drummed by Yoichi Kobayashi that passes a long line of players through standards such as "Oleo," "Blue in Green" and "Tenderly"; Yamashita is the bassist on seventeen of the tracks. His own biography reports that readers of Swing Journal placed him fifth in the acoustic bass category of the magazine's popularity poll for domestic jazz players in 1998 and second in 1999, and a Japanese reference page adds fifth again in 2000.

Japanese biographies of him name two American visitors. One says the pianist Hank Jones noticed him on a television programme and played with him whenever Jones came to Japan; his teaching biography adds that he has played repeatedly with Jones and with the tenor saxophonist Scott Hamilton. No recording with either turned up in the discography databases checked, so these stay his own account rather than something a credit can confirm.

In the 2000s his credits are mostly piano trios and singers. With the pianist Kenichi Shimazu he recorded Symmetrophobia, a piano, cello and bass set taped live at SKYDOOR Art Place Aoyama on August 3, 2000, and in 2003 the two-bassist Double Double Bass Session, on which Shinichi Kato is the other bassist. Mukai brought him into Super 4 Brass (Shiosai ZiZO, January 2001), a four-horn record in which Mukai arranged every piece. In 2004 he appeared as a classical contrabassist at a concert given by his teacher Yasuo Takanishi, and the same year he was the bassist on the singer Asuka Watanabe's Unaffected. Singers fill much of the rest: Marica Hiraga's My Shining Hour with a four-trombone section, Romi Akiyama's Tapestry of Memories (2008), Nobuko Kojima's Stretch Over (2011, produced by Mukai, who also plays trombone), Machiko Miura's Archaic Machiko (2015) and Shinobu Ebihara's A Foggy Day (2021). The pianist Akane Matsumoto gave him a longer-running partnership. She recorded Fallin' in Love with Phineas... (Savoy Jazz, 2008) with him, then made seven albums as the New Roman Trio between 2008 and 2013 for Columbia and Denon, from Bach Jazz and Bacharach Jazz to the seasonal Sakura Jazz and Christmas Jazz and the Korean-pop covers of K-Pop Jazz. In February 2011 she, the drummer Gaku Hasegawa, the guitarist Hironobu Saito, the vocalist Kei Uemura and Yamashita put all five names on Kumuhula Jazz (Overlap Record, OVLC-3), fifteen Hawaiian songs, the closest Discogs comes to showing a release with his name on the front.

He turned 59 on October 9, 2026, and is still working steadily. Live listings for 2026 show him with Shimazu in the Blue Moon club's Vocal JAM series (most recently on October 16, with another date set for November 7), in the alto saxophonist Joh Yamada's quartet (October 10, with a December 6 date listed), and booked for November 2 with the vibraphonist Takashi Oi's Super Vibration at Ginza Swing. In August he played a trio set billed under his and Shinobu Ebihara's names, and in July he was the bassist in a big band at B flat. He teaches in the bass and ensemble departments at the Root's Academy of Jazz in Shinjuku, Tokyo, and has studied bowing with Nobuyuki Tanaka of the Nagoya Philharmonic and with the late Yasuo Takanishi.

Descriptions of Yamashita's playing come back to the same few words: a warm acoustic tone, a big sound, and a style that is steady and bold at once. He studied bowing with Nobuyuki Tanaka of the Nagoya Philharmonic and with the late Yasuo Takanishi, and he has appeared as a classical contrabassist, but the working career is overwhelmingly mainstream jazz: trios behind pianists, quartets behind singers, horn sections and a big band.

The database record bears that out. Most of his credits are on records led by a pianist, a singer or a horn player, and the repertoire on them ("Joy Spring," "Blue in Green," "Wild Is the Wind," "Speak Low") is straight out of the standards book. What the credits also show is range: a New York date with rising American horn players in 1993, a piano, cello and bass trio, a two-bass piano session, a Hawaiian-song project and a piano-trio series of pop covers. He has not released an album under his name alone.