The leaders, who visit Chelsea in their Champions League last-16 second leg on Tuesday with a 2-1 advantage, could see their domestic lead slashed to a point if second-placed AC Milan beat visiting Chievo on Sunday.
Inter's Ghanaian substitute Sulley Muntari had only been on the pitch a minute when he committed a foul. He then handled Giuseppe Mascara's resultant free-kick to concede a penalty and was also sent off for a second yellow card.
Mascara converted the spot-kick nine minutes from time with a cheeky, dinked finish to put the hosts 2-1 up and Jorge Martinez added a third in the 90th minute.
"Inter are the strongest team in the championship but tonight we have shown that when we play with grit and determination we can beat anybody," Uruguayan forward Martinez told Sky TV.
Inter, who have won just once in six league games having appeared to be cruising to a fifth straight scudetto, went ahead early in the second half through Diego Milito but fellow Argentine forward Maxi Lopez equalised on 74 minutes.
SWEET REVENGE
"Until we took the lead we controlled the game then we paid for a lack of concentration," said Inter technical director Marco Branca.
"We knew Catania were enjoying a good moment of form. We weren't thinking about Chelsea, that will be another game."
The win was sweet revenge for tough-talking Catania coach Sinisa Mihajlovic, a former Inter assistant coach who has repeatedly clashed verbally with outspoken Inter boss Jose Mourinho over the last two seasons.
Mourinho was serving the final part of a three-match touchline ban but looked to have produced a masterstroke at halftime following an even first period.
He introduced winger Ricardo Quaresma, who troubled the home defence, and Milito was soon on the scoresheet after the consistently impressive Wesley Sneijder and Samuel Eto'o had combined to set up his 16th league goal of the campaign.
However, just when the close-season buy from Genoa looked to have sealed yet another Inter win, sixth-from-bottom Catania stormed back to inflict the third league defeat of the season on the champions with 10 games left.
