Fresh from a brilliant Champions League victory over Benfica in midweek, the hosts looked determined to make a positive start against their Lombardian opponents.
However, despite dominating large spells of possession in a closely-fought opening half hour, clear-cut chances came at a premium for the Nerazzurri, with Romelu Lukaku heading wide from inside the box and Joaquin Correa seeing a low strike thwarted by Michele Di Gregorio.
Desperate to avoid a third consecutive Serie A defeat on home soil for the first time since 2013, Inter upped the ante before the break in pursuit of a breakthrough.
For all their sustained pressure, a distinct lack of cutting edge in the final third continued to prove problematic for the hosts, as Kristjan Asllani curled a promising free-kick wastefully over the bar, to keep the sides level going into the interval.
Undeterred by their first-half profligacy in front of goal, Inter emerged after the restart eager to break Monza’s resistance, but Di Gregorio again stood firm to deny a close-range header from Lukaku.
Having survived a number of close calls on their own goal, the visitors grew in confidence going into the final half-hour, and came within inches of an unlikely opener themselves, when Dany Mota cut inside Alessandro Bastoni and curled an effort agonisingly over the bar.
With time ticking into the final 20 minutes, Inter boss Simone Inzaghi introduced top scorer Lautaro Martinez in the hope of finding a late winner.
However, it was the visitors who finally found the breakthrough with 13 minutes remaining, as Luca Caldirola planted a superb downward header beyond Andre Onana to give the Biancorossi a shock lead.
Clearly taken aback by Caldirola’s opener, Inter looked to regain some semblance of composure during the closing stages in pursuit of a leveller, however the inspired Monza rearguard held firm to secure a famous triumph.
The result sees Inter miss a golden opportunity to displace rivals AC Milan in the top four, intensifying the pressure on under-fire boss Inzaghi. As for Monza, the Lombardian side rise to 12th, with their Serie A status all but confirmed for another season.
Flashscore Man of the Match: Luca Caldirola (Monza)
See all the stats and player ratings from the match at Flashscore
