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The Austrian 29-year-old can no longer be caught by his closest competitor, Norway's Henrik Kristoffersen, after building a 289 point lead in the overall standings.
Kristoffersen does not compete in the speed disciplines meaning he can only claim a maximum of 200 points at the season's finale in Are, Sweden.
Hirscher, who had already secured a fifth World Cup giant slalom title by winning that event at the Slovenian ski station on Saturday, claimed his fifth slalom globe too.
This was his 12th World Cup victory this season and 57th of his career.
He won in a time of 1min 49.22sec to beat Kristoffersen by a massive 1.22sec, with Switzerland's Ramon Zenhauser third at 1.61sec.