The Norwegian’s event got off to a rocky start. He struggled with the set-up of his Citroën C3 R5 on Friday and ended the first full leg in third.
The tables were turned on Saturday morning when leader Adrien Fourmaux suffered a puncture during the Costa Valle Imagna speed test. Østberg had already leapfrogged title rival Pontus Tidemand at that point and snatched the lead as Sunday beckoned.
A conservative approach during the final day saw Østberg win the category by 25.6secs and lift the title trophy, an accolade he missed out on by one position last year.
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“It's unbelievable, I am really happy,” smiled Østberg at the finish.
“It was such a difficult weekend, with so many ups and downs all through the year. We have won the championship as the underdogs and we have developed the car through the year - just an amazing job by the team to get to this point,” he added.
Tidemand was disappointed to lose out on the victory, but remained positive despite getting a front right puncture during the rally-closing Wolf Power Stage.
“We did what we could and I think the performance was there, but we made bad tyre choices. I am happy to finish and it was really fun,” said the runner-up.
Jan Kopecky was 57.8 sec behind his Fabia team-mate in third after a frustrating weekend, whilst M-Sport Ford Fiesta pilot Fourmaux completed the leaderboard.
Fourmaux’s Friday pace made him an early contender for the Monza win. However, front suspension damage after the Saturday puncture put paid to his efforts and he ended 29min 20.4sec adrift of top spot.