In September 2008, she scaled the summit of women’s chess when she became the 14th Women's World Chess Champion after winning the final of the Women's World Championship (2008) against Yifan Hou by 2.5 to 1.5. In December 2010, she conceded her crown when she bowed out in the third round to finalist Ruan Lufei.

A WGM since age 14, an IM at 16, and a GM at 20, Kosteniuk has featured prominently in various regional and world age title competitions, including winning the Girls U10 European Championship in Herculane in 1994, the Girls Under 12 European Championship in Rimavaska Sobota in 1996, and the Girls Under 12 World Championship in Minorca in 1996. In 2001, at the age of 17, she was runner up to Zhu Chen for the Women’s World Championship. In 2000 and 2004, Kosteniuk was second in the Russian Women’s Championships but went one better in 2005 when she won that Championship with a score of 9/11 and a 2691 performance rating. Other prominent tournament successes include winning the First European Women’s Championship that was held in Dresden in 2004, and equal second behind Koneru Humpy in the 2005 North Urals Cup. In 2010 she came fifth in the Russian Women's Championship and in 2011 she won the Moscow Open 2011B - Young GMs with 7/9.

Kosteniuk became the inaugural Fischerandom (Chess960) Women's World Champion in 2006, and successfully defended her title in 2008 against Kateryna Lahno. There did not appear to be a Fischerandom world championship in 2010, and Kosteniuk is therefore still the women's world titleholder for this form of chess.

She has captured the public's eye by appearing in a Russian film, doing many product promotions, fashion modeling, and appearing in fashion magazines. She produces a popular series of chess podcasts, and maintains a chess website http://www.kosteniuk.com/