He said he was wearing jeans to a lunch meeting and “didn’t even think about” changing them for another pair of pants as he headed to the tournament.
He turned up in a shirt, jacket, dark jeans and shoes and played a few rounds before being asked to change.
When his offer to change the next day was rejected, Carlsen said it “became a bit of a principle for me”.
In a statement, FIDE confirmed the 34-year-old had been fined $200 (£159) and said its rules were applied “impartially”. They cited a case where another player was fined the same day before changing shoes.
Carlsen is a five-time world chess champion and maintains a leading position in the sport.
The Norwegian has long been regarded as a chess player in the chess world since becoming a grandmaster – the highest title in chess – at the age of 13.
In a settled dispute with opponent Hans Niemann, Carlsen withdrew from the 2022 tournament after Niemann beat him before accusing his American rival of cheating.
Nieman denied the allegations and even said he would “undress” to prove his innocence.
Last August, the couple settled the lawsuit for $100m (£79m).
