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
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
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.