A 2-1 victory over city rivals AC Milan has given Inter their 20th Serie A title on a raucous night at San Siro - with five games of the season still to go.
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Inter's success, which caps an utterly dominant season in the league, breaks a tie with Milan and gives the Nerazzurri a coveted second star of the team shirts.
Italian teams are able to sport one gold star for each 10 top-flight titles won. Only Juventus have won more, with three stars after a record 36 championships.
Not even the pouring rain could dampen the spirits of Inter's fans as they joyously reclaimed city bragging rights.
"I told the guys, 'We find ourselves in a situation that's never happened before'," Inter captain Lautaro Martinez said.
"We needed to take advantage of this opportunity, because look around - the entire stadium is ours now."
Francesco Acerbi and Marcus Thuram scored to give Inter a 2-0 lead by early in the second half, but Fikayo Tomori's late goal for Milan threatened to delay the celebrations.
But Inter held on for the win that stretched their lead to an insurmountable 17 points over second-placed Milan, with 15 left to play for.
Simone Inzaghi won his first league title as a coach.
"Winning the league with five matches left - more or less won with nine matches left, today making it mathematically certain - in a derby, with Milan the home team, that is something that will remain with us," Inzaghi said.
"Everyone gave their very valuable contribution so that we could reach goals like this."
Inter had won the past five derby matches, including both legs of the Champions League semi-final last year and a 5-1 humiliation of Milan earlier this season in Serie A.
They took the lead in the 18th minute when Benjamin Pavard nodded on Federico Dimarco's corner for an unmarked Acerbi to head in from close range.
Clinching the crown moved closer still three minutes after halftime, when Thuram ran onto a long Alessandro Bastoni ball over the top and feinted around his marker to cut in from the left along the edge of the area and drill into the bottom corner.
There was a tense finale after Milan pulled one back 10 minutes from time.
Inter goalkeeper Yann Sommer did brilliantly to parry Mattia Gabbia's header onto the post but Tomori nodded in the rebound.
As tempers flared, a scuffle broke out in stoppage time and Milan's Theo Hernandez and Inter defender Denzel Dumfries were sent off.
Milan's Davide Calabria was also shown a red card moments later, following another scuffle.
Bologna consolidated fourth place with a 3-1 win at Roma - seven points behind in fifth, with a game in hand - to inflict Daniele De Rossi's second league defeat as coach of the capital club.
Australian Associated Press
