Andrea Caracciolo scored the only goal as promoted Novara completed a remarkable double over their opponents, having beaten them 3-1 in September in the game which prompted Inter to sack Gian Piero Gasparini after five games.
Inter, thumped 4-0 at AS Roma a week ago, have taken only one point from their last four matches, stopping their Claudio Ranieri-led revival in its tracks.
Fifth in the table, they have dropped 11 points behind leaders AC Milan, after having cut the gap to five a month ago with their win in the Milan derby.
Milan, 2-1 winners at Udinese on Saturday, have 47 points from 23 games with unbeaten Juventus two behind in second with two games in hand.
Juve's game at Bologna scheduled for later on Sunday was postponed because of bad weather, as was Parma's game at home to Fiorentina.
Caracciolo broke the deadlock at a freezing and half-empty San Siro when he curled a left-foot shot past Julio Cesar in the 56th minute.
The visitors survived a torrid last few minutes after Ivan Radovanovic was sent off in the 80th minute, receiving a second yellow card for hoofing the ball upfield after his side had conceded a free-kick.
Novara goalkeeper Samir Ujkani did well to stop Wesley Sneijder's effort from the free-kick, before the Dutchman crashed another shot against the underside of the crossbar in the dying seconds.
Albanian Ujkani then produced a superb stop to deny Giampaolo Pazzini at point-blank range with virtually the last kick of the game.
Novara pulled level with 19th-placed Cesena on 16 points, two behind Lecce.
Atalanta drew 0-0 with Lecce while Catania thrashed Genoa 4-0 in Sunday's other games.
Francesco Lodi's penalty gave Catania an early lead before Pablo Barrientos scored twice in the second half and his fellow Argentine Gonzalo Bergessio completed the scoring.
