Gerard Mortier, a visionary opera company leader whose bold theatricality and updating of the canon helped define the art form’s modern history, died on Saturday at his home in Brussels. He was 70.

The cause was pancreatic cancer, said Simon Bauwens, his personal assistant at Teatro Real in Madrid.

Mr. Mortier was that company’s artistic director from 2009 until last year, when his title was changed to artistic adviser in a tussle with the Spanish government over his successor after he announced in September that he was being treated for cancer.

It was a characteristic dispute for a man who always relished a battle during a four-decade career at the helm of some of the world’s most important opera companies, including the Salzburg Festival and the Paris Opera.

Sometimes the grounds were artistic, as in the furor over his farewell production after 10 years in Salzburg: a 2001 “Die Fledermaus” taking aim at the Austrian government and featuring drugs and Nazi thugs.

Sometimes they were financial, as when the Théâtre de la Monnaie in Brussels, which he led from 1981 to 1991, went into debt over his lavish renovation of the opera house, complete with a floor by the American Minimalist artist Sol LeWitt.

And sometimes they related to Mr. Mortier himself, as when he and New York City Opera parted ways in 2008, a scant year and a half into his tenure and before he had even arrived to take up the post full time.

These clashes were less tantrums than expressions of his bracing, intellectually charged vision of opera and his disdain for the decorous irrelevance often associated with it. In the summer of 2011, reflecting in an interview on a raucous Madrid production of Karol Szymanowski’s “King Roger” the previous fall, he said with a smile: “It was an enormous scandal, and it became an enormous success. On opening night, I said:‘ Now we are really international. People aren’t sleeping at the end.’ ”

Gerard Alfons August Mortier was born on Nov. 25, 1943, in Ghent, Belgium, where his parents owned a bakery in a working-class neighborhood. His mother and grandmother took him to the opera as a child, and at home he would stage Mozart’s “Magic Flute” in a puppet theater.

He said he had acquired his taste for controversy at a boarding school run by the Jesuits, who had him and his classmates read iconoclastic writers like Marx, Nietzsche, Ibsen and Sartre. In 1968, as students protested throughout Europe, Mr. Mortier formed a group of young opera fans that loudly jeered productions it judged overly conservative.

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