Spiess, who enjoyed a 14-year career before damaged vocal chords forced him to give up singing, made his debut as the Duke of Mantua in Rigoletto in the Romanian city of Galati in 1962; he appeared as Cavaradossi in Tosca at the Bucharest Opera two years later.
In 1967 Herbert Von Karajan invited him to the Salzburg Festival to sing Dmitri in Mussorgsky's Boris Godunov. The next year he joined the Zurich Opera, where he remained for some years. At the Metropolitan Opera his roles included Florestan in Fidelio, Manrico in Il trovatore and Canio in Pagliacci; with the San Francisco Opera he sang Calaf and Cavaradossi in Tosca.
Spiess was born in 1938 in Cluj. His vocal talents first came to light when he sang in the choir of the factory in Brasov where he worked after leaving school.
Other notable successes included Smetana's Dalibor in Vienna and Don Jos_ in Bizet's Carmen at the Bregenz Festival. He recorded Mahler's Das Lied von der Erde in 1975 with Janet Baker, released four years ago on BBC Legends.
After retiring from the stage, he became minister of culture in Romania's post-Communist government in the early 1990s, and was director of the Romanian Opera from 2001 until last year.
According to the Telegraph Spiess died while hunting in a forest in southern Romania. He was 67.