Has a woman ever defeated a reigning undisputed World Chess Champion in classical chess? I know that the great Judit Polgar defeated Kasparov, but he was no longer World Champion at that point. She defeated Anand, but he was not yet World Champion at that point. She defeated Kasimdzhanov, but his championship wasn't undisputed because of the FIDE-PCA split. And so on.

Has a woman ever defeated a reigning undisputed World Chess Champion in classical chess?
Sorry, but the answer is "no".

I used ChessBase and the Mega 2020 database to search various combinations of players, focusing primarily on Women's World Champions, Judit Polgar, Hou Yifan, Vera Menchik from the 1920's and 1930's, Lyudmila Rudenko, Elisaveta Bykova, Olga Rubtsova, Nona Gaprindashvili, and Maia Chiburdanidze. I stopped after Maia Chiburdanidze with the exception of Polgar and Yifan since I would have remembered a victory by the more recent Women's World Champions starting with Xie Jun next. With the exception of Vera Menchik, who played Capablanca a number of games, none of the old Soviet women ever played any of the world champions. 

Given that you were looking for a defeat of a sitting world champion in classical, no woman has ever done that yet.

The only ones to ever beat men, who already had been world champions, were Judit Polgar's victories over Karpov in classical at Hoogeveen in 2003, and Chorus in 2003, and the Champions League in 2007 (still the only victories by any woman in classical over even a former World Champion), and 8 wins in rapid, and a few blitz. Also, she beat Kasparov in rapid at the World Rapid Championship in 2002. She also had one rapid victory over Magnus Carlsen in 2012 in Mexico City, but that was a year before he won the title.

Hou Yifan had two wins over Kramnik at the Media Kings Rapid in 2016. She also has had some victories in blitz.


  After a bunch of back-and-forth comments below, I was asked to add my
  methodology to the answer, which I agreed to do. I do not claim my
  answer to be worthy of a scientific study and the scrutiny that would
  be associated with it. I was just trying to give a reasonably educated
  answer to the question. Whether I missed a rapid game against a former
  World Champion in my list, that is possible, but that was also just
  supplemental information I provided, and was not really part of the
  answer to the OP’s question since the question was officially only
  about classical chess. I would also take classical chess to mean
  over-the-board in a tournament since for it to mean anything, it had
  to be serious chess (this is not to be confused with relays of OTB
  tournaments, where the games are not really being played on the
  Internet).
  
  First, it is important to understand that if no woman has ever beaten
  a sitting World Champion in classical chess, it would be asking to
  prove a negative, so you can always some up with a “what if” situation
  that I did not search for. I had to draw the line somewhere, and I
  drew the line based on my searches of the best women players of their
  times (and all the world champions of their eras they may have faced),
  my fairly extensive knowledge of chess history, and 40 years of
  reading MANY sources on a once monthly (before the Internet, when all
  I was “Chess Life” and “New in Chess” magazine), and since the
  Internet, I read an average of three sites per day, every day
  (currently chessbase.com, chess.com, and chess24.com, and twic and
  chessninja in the past).
  
  It is also important to know, and this is where the history comes in,
  that the best women of the generations prior to the Polgar sisters,
  were MUCH weaker, often only in the 2300-2400 range, but it was also
  before the ELO system existed, so that is an estimate. I also know
  that back then, men simply did not play women virtually at all…Vera
  Menchik was the exception.
  
  All of the major advances in women’s chess strength to the Grandmaster
  level have come during my lifetime, when I was active playing, and
  actively reading the sources I referenced above, so I also ask for a
  little common sense: This would have been HUGE news, as was Polgar
  beat Kasparov in RAPID. I cannot find anything on Google about a woman
  beating a sitting World Champion in classical chess. I would not have
  missed it.
  
  Lastly, World Champions since Fischer have been 2700+, and there are
  only two women, who reasonably have had a shot at beating a World
  Champion in classical, and a lot of that is due to opportunity and
  invitations to elite events: Judit Polgar, and Hou Yifan, so you
  really do not have to look much further than their games, which limits
  the searching a lot.