The Brazilian substitute silenced the San Siro crowd with a wonderful last-minute equalizer following Fredy Guarin's opener for the hosts.
The 27-year-old Brazilian – who had not scored a goal for Sampdoria since joining from Cluj in January 2012 – hit a left-footed effort into the bottom right corner that left Inter goalkeeper Samir Handanovic with no chance.
Colombian Fredy Guarin had earlier given Inter the lead in the 18th minute, producing a smart finish after excellent build-up work from Ricky Alvarez.
Guarin then hit the post in the second half, but Sampdoria also enjoyed its fair share of chances, while Eder had a goal ruled out for offside after the break.
But it snared its hard-earned point at the death as Renan's effort stunned the crowd at San Siro, which included new Inter president Erick Thohir, watching the club for the first time since his takeover in October.
Captain Javier Zanetti came in for Inter, with the veteran making his first start for the club since suffering an Achilles injury in April, while Sampdoria drafted in Eder and Roberto Soriano.
The visitors started brightly, quickly settling into a rhythm, and just after Eder fired over with a bicycle kick, Manolo Gabbiadini volleyed wide.
Alvarez was proving a threat down the left for Inter though, and his quick feet and vision set up the opener, as he skinned Gabbiadini before picking out Guarin at the far post.
Guarin took a touch and, from an acute angle, fired past Sampdoria goalkeeper Angelo Da Costa, who could not keep out the effort despite getting hands to it.
Buoyed by the goal, Jonathan and Esteban Cambiasso had attempts, while Nicola Pozzi showed good skill to gather and turn at the other end, but his shot lacked power.
Daniele Gastaldello was taken off on a stretcher just before halftime, replaced by Vasco Regini, in a further blow for Sampdoria, which could've considered itself unlucky to be behind at the break.
Inter started brightly after halftime as a superb Guarin ball nearly found Saphir Taider, and the Colombian thrashed a powerful effort against the post in the 51st minute.
Eder found the net for Sampdoria at the other end, but the goal was ruled out for a clear offside, as Mateo Kovacic replaced Taider just after the hour mark for Inter, which looked to kill the game off.
The visitors pressed on in their search for an equalizer though, as Inter goalkeeper Samir Handanovic spilled an Eder free kick, but Cambiasso was on hand to clear the rebound.
Lorenzo De Silvestri then headed over from a corner for Sampdoria, but they did get their leveler, thanks to Renan's effort in the dying stages, denying Inter the chance to move third in Serie A.