Yoon Sun Choi is an improvising vocalist and pianist who was born in Korea, grew up in Ontario and has been based in New York since 2000. Her long partnership with the pianist Jacob Sacks has produced the albums Soulmates (2002) and Imagination: The Music of Joe Raposo (2008) and, in 2025, Memory Ghosts, and she has also sung in the groups 4inObjects and Rob Garcia's Sangha, in the Tri-Centric Orchestra's choir and on Nate Wooley's Seven Storey Mountain VI.

Her Korean basis is thin, and it is stated here as it is. The sources give no birth year, no birthplace within Korea and no Hangul form of her name; her releases print it in Latin letters as Yoon Sun Choi, and early Canadian press shortened it to Yoon Choi or Yooni Choi. A Toronto Star profile of September 2002 says she was born in Korea and raised in Whitby, Ontario, and a 2025 concert listing from Piano on Park describes her as born in Korea, raised in Canada and now an American. Her own biography calls her a Korean-Canadian vocalist. No Korean release, festival date or Korean press coverage is documented for her; her whole working life is in Canada and the United States. She is a different person from Youn Sun Nah, the Seoul-trained singer based in France, and from the K-pop performer who is also named Choi Yoon-sun.

Her training was in Ontario. Her biography lists a bachelor of music in classical piano and composition from Queen's University in Kingston and a bachelor of music in voice and jazz performance from the University of Toronto, with voice studies under Thomas Schilling and piano studies under the late Sofia Rosoff. The Toronto Star adds that she sang in an a cappella jazz choir called Voice Deco in her student years, studied at Berklee College of Music and finished her University of Toronto degree in vocal jazz in 1997. Before she left for New York, Toronto listeners knew her from standards sets at clubs such as The Rex and from The Source, a jazz-pop group.

New York changed the direction of the music. The Toronto Star reported in September 2002 that she had spent the previous eighteen months in the city, immersed in its avant-garde and new-music scene, and that the quartet she brought to the Senator in Toronto, with the pianist Jacob Sacks, the bassist David Ambrosio and the drummer Dan Weiss, played mostly original music and vocalese. She described a project of "modern standards", songs written in roughly the last twenty years, from Radiohead to Peter Gabriel to Bjork, treated as Ornette Coleman treated a melody. The same circle became 4inObjects, a quintet of Choi, Sacks, Ambrosio, Weiss and the trombonist Jacob Garchik. Its self-titled album (Yeah Yeah Records, 2006) was recorded live about three years earlier, according to the All About Jazz review by Rex Butters, and pairs her originals "Back to You" and "Tunnel Mountain Drive" with Radiohead's "Airbag" and Burt Bacharach's "Close to You". Ty Cumbie's All About Jazz review of March 2006 called her singing strong and direct and noted that the band was a regular at the 55 Bar. In June 2002 she was already the vocalist when the drummer Rob Garcia introduced his group Sangha at the Cornelia Street Cafe, according to David R. Adler's column in All About Jazz; the group's album Heart's Fire (Connection Works, 2007) credits her with vocals.

Soulmates, the first record by the Choi and Sacks duo, was recorded without edits or overdubs on May 4, 2002 at the CBC studios in Toronto and released that autumn on Yeah Yeah Records. It was reviewed in the Ottawa Citizen (four stars, Peter Hum, September 28, 2002), which heard a deep, rich voice and earthy scatting, and in Cadence, where David Dupont compared the duo's mix of art song and noir to the work of Jeanne Lee with Ran Blake. The duo's own page dates their collaboration from about 2000 and lists Carnegie Hall, Barbes, Birdland, The Stone, The Jazz Gallery, the 55 Bar and Cornelia Street Cafe among their stages. In 2005 and 2006 they ran a music series in the East Village called The Restless Spirits, inviting guests such as Mat Maneri, Ben Monder, Thomas Morgan, Eivind Opsvik, Angelica Sanchez and Matt Wilson. Back in Ontario in September 2003 she sang the role of the law student Colette Chan in Québécité, the jazz opera by the pianist D.D. Jackson and the poet George Elliott Clarke that was commissioned for the tenth anniversary of the Guelph Jazz Festival; the Guelph Mercury singled out her scene in her parents' bar.

Imagination: The Music of Joe Raposo (Yeah Yeah Records, 2008) set the songs that Raposo wrote for Sesame Street and The Muppets, among them "Bein' Green", "Happiness Hotel", "Sing" and the title song. Ted Gioia's review on Jazz.com, dated June 10, 2008, found the songs well beyond children's music in her hands, praised Sacks as a daring accompanist who pushes the singer, and named the record one of the best jazz vocal albums of its year. The duo's later recordings include I Should Care, a set of Great American Songbook standards named in Piano on Park's listing (no release details were found), and Memory Ghosts, recorded on October 18, 2019 at Oktaven Audio and issued on Yeah Yeah Records on January 25, 2025, the day of its release concert at the Church of the Advent Hope on East 87th Street. It has thirteen tracks, some of them under a minute, among them "Strange Fruit", "My Man's Gone Now" and "Political Science"; Piano on Park says it mixes new original material with songs by Annette Peacock and Randy Newman. Chet Doxas co-produced it with the duo, Eivind Opsvik mixed and mastered it, Sacks designed the cover and Choi took the cover photograph. On April 6, 2022 the duo played a Carnegie Hall "Duos" bill at Weill Recital Hall, shared with the saxophonist Randal Despommier and the pianist Jason Yeager.

Beyond the duo she has been a steady presence in New York's improvising community. On the Tri-Centric Orchestra's 2016 release she is credited in the choir for two works recorded at Roulette in Brooklyn in 2013, and her biography names the composer Sarah Weaver and her ensemble among her regular collaborations. She is one of three vocalists, with Megan Schubert and Mellissa Hughes, on Nate Wooley's Seven Storey Mountain VI (Pyroclastic Records, 2020), recorded on November 24, 2019. Martin Schray's review for the Free Jazz Collective describes a continuous work of about 45 minutes in which the singers open by humming Peggy Seeger's "Reclaim the Night" and return at the end to sing it. She is also one of the voices on Dan Loomis's song cycle Job's Trials (2020), alongside Song Yi Jeon, and on the 2012 folk album The Bridge by Escapist Papers. Her own site describes a quartet called Owls at Night, with the violinist Dana Lyn, the bassist Scott Colberg and the drummer Vinnie Sperrazza, in which she sings, plays piano and writes all the music. A March 2024 post by Sperrazza describes another group, a quartet with Sacks and the bassist Thomas Morgan that she assembled in summer 2020 for a porch concert in Ditmas Park, Brooklyn, among four players who had known one another since the early 2000s.

In October 2020 she gave a video interview to Papergirl Brooklyn about living in Flatbush and performing in public spaces, and her website also shows her work as a photographer.

She is a singer who treats voice and piano as equal partners in an improvising duo. Her own account of the 2002 repertoire is of "modern standards": recent songs reharmonized and never played the same way twice, with melodies that stay simple while the treatment is free. In that interview she named Betty Carter, Norma Winstone and Kenny Wheeler as particular inspirations. Her biography describes an uncompromising, freewheeling style, and her work spans wordless scat, plain-spoken original songs, children's songs reimagined and standards.

Critics place her at the exploratory edge of vocal jazz. Cadence heard in Soulmates a singer with a fine alto whose plain, journal-like lyrics are set into the quiet, uncluttered piano of Sacks. Gioia wrote that Sacks challenges her aggressively on Imagination, which he considered rare in a voice and piano record, and Henkin, reviewing the duo's Raposo set at 5C Cafe, described her moving between a traditional jazz delivery, a sultrier approach and avant-garde vocal gymnastics. Piano on Park's listing says the duo has stood at the exploratory forefront of improvised voice and piano for 25 years.

Her catalog also reaches outside conventional jazz. Wooley's Seven Storey Mountain VI is a long experimental work and the Tri-Centric Orchestra choir is a large-ensemble new-music setting, while the Escapist Papers record is folk. The case for calling her primarily a jazz artist rests on her jazz degrees and early work in Toronto, the jazz clubs and festivals the duo has played, and the jazz press that reviewed Soulmates, 4inObjects and Imagination.