The man who was once first in line to the throne of the former Austro-Hungarian empire which covered most of central Europe, died at his home in Pöcking on Lake Starnberg in Bavaria, his spokeswoman said in a statement.
The son of emperor Karl I, who reigned for only two years before the empire disintegrated, Habsburg was born on November 20, 1912 in Reichenau an der
Rax in eastern Austria.
Known abroad as Otto von Habsburg, this elegant man with large glasses and a big smile was just Otto Habsburg-Lothringen in Austria, after the state abolished his family's titles and confiscated their property in 1919.
But he found a calling in the European project, heading the International Paneuropean Union for over 30 years and serving as an elected member of the European parliament from 1979 to 1999 for Bavaria's conservative CSU party. He held Austrian, German and Hungarian citizenship.
An ardent anti-Communist, Habsburg organised in August 1989 the now-famous "Pan-European picnic" in Sopron, near Hungary's border with Austria, during which some 700 East Germans were able to escape to the West, a few months before the Berlin Wall fell on November 9.
He always campaigned for a unified Europe based on Christian values and for a greater integration of eastern countries into Europe.
Forced into exile with his family after the empire fell in 1918, Habsburg spent time in Switzerland, on the Portuguese island of Madeira, Spain, Belgium, France and the United States, and studied political and social sciences at the Catholic University of Leuven in Belgium, graduating with a doctorate in 1935.
In the 1930s, he openly opposed the Nazi party, joining the Austrian resistance after the 1938 Anschluß and helping thousands of Jews flee the country at the beginning of World War II.
His actions prompted the Nazis to come after him but Habsburg later said it was his duty to be politically involved.
He said he became "a European patriot in the very depths of his soul" after Nazi Germany occupied Austria.
In 1951, Habsburg married the German princess Regina von Saxe-Meiningen und
Hildburghausen, with whom he had seven children.
He reluctantly renounced all claims to the Austrian throne in 1961, five years before the state repealed so-called anti-Habsburg laws, adopted after World War I, which effectively banned all members of the former imperial family from setting foot in Austria.
A student of languages, Habsburg wrote some 40 books on history and politics in German, French, Spanish and Hungarian.
His funeral will take place in Vienna on July 16, his spokeswoman said Monday. Habsburg will be buried in the imperial crypt in Vienna, Cardinal Christoph Schönborn told the Catholic news service Kathpress.