is a Japanese former professional boxer. She is a two-weight world champion, having held the WBC female atomweight title from 2008 to 2015; the WBA female atomweight title in 2015; and the WBC female strawweight title in 2017. Koseki made a record-breaking seventeen consecutive title defenses of her WBC atomweight title. Koseki is a member of the International Women's Boxing Hall of Fame.

Career Koseki was inspired to take up boxing after watching Hiroshi Kawashima on TV. Koseki won the inaugural All-Japan women's amateur championships in 2003, as well as the second edition in 2004. After winning a third national amateur title in 2007, she decided to turn pro as women's boxing was not yet accepted as an Olympic sport.

Koseki won her first three bouts. She went on to challenge Winyu Paradorngym for the inaugural WBC atomweight world title. Winyu won the fight by unanimous decision. In her next fight, Koseki moved up two weight classes to challenge for the WBC light flyweight title, but lost again by unanimous decision. In August 2008, Koseki rematched Paradorngym, and won the title with a round 2 knockout at Korakuen Hall. In August 2014, Koseki made her fourteenth consecutive defense, beating Denise Castle with a round 8 technical knockout. With the win, Koseki broke Yoko Gushiken's Japanese national record of 13 consecutive successful world title defenses. In October 2015, Koseki beat WBA champion Ayaka Miyao by unanimous decision to make her sixteenth defense and unify titles.

Koseki won the WBC minimumweight title on 17 December 2017, defeating reigning champion Yuko Kuroki by decision.

She retired on 29 January 2018, at the age of 34.

In October 2025, Koseki was named among the inductees for the 2026 International Women's Boxing Hall of Fame class.

Professional boxing record

ResultRecordOpponentTypeRound, timeDateLocationNotes27Win24–2–1Yuko KurokiUD1017 Dec 201726Win23–2–1Chie HiganoTKO4 (10), 11 Nov 201625Win22–2–1Rungnapha KaewkrachangTKO1 (8), 13 Aug 201624Win21–2–1Ayaka MiyaoUD1022 Oct 201523Win20–2–1Aisah AlicoKO2 (10), 19 Feb 201522Win19–2–1Denise CastleTKO8 (10), 2 Aug 201421Win18–2–1Angor OnesongchaigymTKO9 (10), 3 Mar 201420Win17–2–1Nora CardozaUD1014 Nov 201319Win16–2–1Huh Eun-youngTKO6 (10), 28 May 201318Win15–2–1Maria del Refugio JimenezUD103 Mar 201317Win14–2–1Saemi HanagataUD1017 Dec 201216Win13–2–1Jujeath NagaowaUD1019 Jun 201215Win12–2–1Mami Ito10 (10), 30 Nov 201114Win11–2–1Krikanok IslandmuaythaiTKO5 (10), 8 May 201113Draw10–2–1Masae Akitaya3 (10), 6 Jun 201012Win10–2Shin Gun-jooUD1027 Mar 201011Win9–2Teeraporn PannimitUD1029 Nov 200910Win8–2Nao IkeyamaUD102 May 20099Win7–2Kim Hye-minUD108 Dec 20088Win6–2Chirawadee Srisuk2 (10), 11 Aug 20087Win5–2Petchsifah Sithkrumad2 (6), 9 Jun 20086Win4–2Mayumi KuboUD49 May 20085Loss3–2Siriporn ThaweesukUD1019 Nov 20074Loss3–1Chirawadee SrisukUD1031 Aug 20073Win3–0Singsamaoy Sitnhongalampoon67 Jul 20072Win2–0Teeraporn PannimitUD616 Jun 20071Win1–0Yupin Ha Payak426 May 2007

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