The challenger had his runner locked out.
His 27th move was a mistake that beginners usually make.
Magnus Carlsen only had to move a pawn forward and then pick up the piece that could neither escape nor defend.
After his third win within four games, the Norwegian is now 6-3 at the World Chess Championship in Dubai and is about to defend his title.
Jan Nepomnjaschtschi had already lost the eighth game on Sunday due to a mistake that was unusual at World Cup level.
On Monday, when there was no match, he had his hair cut and Sergei Karjakin flown in from Moscow.
The Russian began the ninth game not with the king's pawn but with an English opening.
In the middle game he captured a pawn, but retained several weaknesses.
Perhaps the only way the peasants would not return was to cost the runner instead.
After Carlsen locked him in, Nepomnyashchi stayed in his quiet room backstage for 18 minutes before returning to the board.
The Polish grandmaster Grzegorz Gajewski tweeted: “I think there is no chess player who doesn't feel sorry for Jan now.” He dragged the game until move 39 before giving up.