Ms. Neff, 76, who was also known as Hildegarde Neff (as material for the 1955 musical indicates), introduced Porter's "Paris Loves Lovers" (with Don Ameche), "It's a Chemical Reaction, That's All," "Without Love" and "As On Through the Seasons We Sail."
AP reported that officials at the Heckeshorn Lung Clinic said Ms. Knef suffered from emphysema and died of a lung infection. She had emergency lung surgery in 2001.
The German-born Ms. Knef appeared in the first German film following World War II, "Murderers Among Us," in which she played a former concentration camp inmate. The native of Ulm took classes at Babelsberg Film Institute and was a stage actress in Berlin when the war ended.
AP reported that Ms. Knef reportedly turned down a Hollywood studio contract after being told she would have to change her name and say she was Austrian, not German.
A nude scene in the 1951 German film, "The Story of a Sinner," shocked the Roman Catholic Church. She also appeared as Pirate Jenny in a film version of "The Threepenny Opera," and appeared in "The Snows of Kilimanjaro."
Ms. Knef became a concert singer in 1960s and penned an autobiography. She is survived by her husband, Paul von Schell, and her daughter, Christina Antonia.