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.