NEW YORK — Johan Botha, the South African tenor whose bronzed voice sailed with ease through some of the most difficult roles in opera over the last two decades, died Thursday in Vienna. He was 51.

Raphaela Hodl, a spokeswoman for the singer, confirmed his death, adding that Mr. Botha had had a “severe illness.’’ The Associated Press reported that he had been treated for cancer. His last performance was on Aug. 13 in a concert in Cape Town that benefited the Cancer Association of South Africa.

Mr. Botha appeared at the world’s major opera houses, including the Metropolitan Opera in New York, the Teatro alla Scala in Milan, the Royal Opera House in London, and the Vienna State Opera. With a voice that penetrated through enormous orchestras while remaining sunny and flexible, he sang, with little apparent effort, notoriously troublesome-to-cast parts in works by Wagner, Strauss, and Verdi.

He was the rare tenor who could make it through Verdi’s “Otello’’ unscathed, and his voice shone through the treacherous lengths of Wagner’s “Parsifal,’’ “Lohengrin,’’ and “Die Meistersinger von Nurnberg.’’ Strauss in particular made almost sadistic demands on his tenors in works like “Daphne.’’ But in 2005 Mr. Botha sang Apollo in that opera “with clarion sound and virile phrasing,’’ Anthony Tommasini wrote in The New York Times.

Dominique Meyer, the Vienna State Opera’s director, said in a statement that Mr. Botha, who sang more than 200 performances with the company since his debut in 1996, was “gone much too soon.’’ He announced that the company would raise a black flag in Mr. Botha’s honor and that this Saturday’s performance of Puccini’s “Turandot’’ would be dedicated to him. Mr. Botha had been engaged to sing in that performance before having to cancel because of his illness.

Born near Johannesburg, Mr. Botha was the son of a postmaster and postmistress. He sang from an early age, mimicking the voices on his father’s old records. During his compulsory military service he performed with the choir of the South African Air Force.

He began as a bass-baritone while studying at the opera school at the Pretoria Technikon, but, discovering that his high notes came more easily than low ones, he reinvented himself as a tenor.

Mr. Botha moved to Europe in 1990; one of his first jobs there was in the chorus of the Wagner-focused Bayreuth Festival in Germany, where 20 years later he would be acclaimed as Siegmund in “Die Walkure.’’ He made a breakthrough debut at the Paris Opera in 1993 in “Madama Butterfly,’’ and first came to the Met in 1997 in “Pagliacci.’’

He leaves his wife, Sonja Botha, and their two sons.

A large man — “I’ve done every diet you can think of,’’ he once told the AP — Mr. Botha could be a stolid, if genial, presence onstage.

But his final performances at the Met, in the vast title role of Wagner’s “Tannhauser’’ last year, found him both singing and acting with new dramatic commitment. Without losing his trademark burnished tone, his “Rome Narration’’ late in the opera bristled with the desperate intensity of a man who had been to hell and back — and lived to tell the tale.