Argentina striker Diego Milito finished coolly on 57 minutes after great work from compatriot and captain Javier Zanetti to leave team mates in tears at full time and controversial coach Jose Mourinho pointing to the sky.
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Second-placed AS Roma won 2-0 at Chievo thanks to Mirko Vucinic and Daniele De Rossi and would have been champions had the final games of the season finished at half time.
Inter topped the final table on 82 points, two above Claudio Ranieri's side, and next meet Bayern Munich in Saturday's Champions League final in Madrid having also won the Italian Cup by beating Roma this month.
"The motivation of this squad has been to put all our force into every match of this championship. In the end the best team has won," 36-year-old talisman Zanetti told Sky TV.
Inter's title triumph was the perfect present for club president Massimo Moratti, celebrating his 65th birthday. Whether Mourinho is his coach for his next birthday remains to be seen with the president admitting before kickoff he does not know what the former Chelsea boss will do after Saturday.
Media speculation is rife that the Portuguese will move to Real Madrid given he has openly acknowledged he is unhappy in Italian football and has repeatedly said he wants to coach a big Spanish club having won titles in England and Italy.
Mourinho joked after Inter's Cup final win that Roma would pay Siena to beat his side.
Siena owner Massimo Mezzaroma, who comes from Rome and whose name translates as "half Roma," laughed off the remarks but his relegated team showed real desire to hurt Inter.
GIANT FLAGS
Siena's Albin Ekdal missed a glorious early chance with the Inter defence all at sea.
Inter settled into their rhythm, though, roared on by a majority away crowd in Tuscany, with surprise starter Mario Balotelli acrobatically rattling the bar and Milito going close.
Further north Chievo's stadium in Verona was filled with the red and yellow colours of Roma fans, who had peacefully passed Inter supporters going in the other direction on the motorway.
The atmosphere exploded with wild cheers five minutes before the break when Vucinic fired in from just inside the area.
De Rossi then blasted in from distance on the stroke of halftime to make it 2-0 and prompt crazed away fans to wave their giant flags in the hope Siena could hold out.
Siena were denied a penalty early in the second period when the ball brushed Thiago Motta's arm but Inter were soon in the ascendancy again and Milito finally broke through wit
