Samir Handanovic's late own goal secured Cagliari a third straight Serie A win and sent Inter Milan spiralling to a third successive defeat in all competitions.
Home goalkeeper Handanovic inadvertently forced the ball over his own goalline in the 85th minute to seal a 2-1 victory for the visitors after Cagliari striker Federico Melchiorri had cancelled out Joao Mario's opener.
Inter followed up a surprise 3-1 defeat to Sparta Prague in the Europa League by losing 2-1 at home to Roma last Sunday and the pressure is mounting on head coach Frank de Boer following this latest setback.
It began badly for Inter when Mauro Icardi fired a 24th-minute penalty wide after he had been fouled by Cagliari defender Bruno Alves.
Icardi and Joao Mario both spurned further first-half chances and the latter was inches wide with an effort just before the break, while Antonio Candreva's low shot was saved by Cagliari keeper Marco Storari.
Cagliari began the second period on the front foot, with Davide Di Gennaro and Melchiorri both threatening before Marco Sau's low shot forced a fine save from Handanovic.
Joao Mario pounced from close range to fire Inter ahead after his initial shot had been blocked by Storari in the 55th minute.
Di Gennaro was denied again by Handanovic soon after, but Cagliari were level in the 71st minute when Melchiorri lashed home a low a low shot from inside the area.
Melchiorri's header and Marco Borriello's angled drive went close to snatching the visitors victory, but they were not to be denied with five minutes remaining.
Melchiorri fired a low cross-cum-shot back across goal following a corner and Handanovic could only carry the ball over his goalline as he dashed back to try and clear the danger.