Duke of Sparta (Greek: Δοὺξ τῆς Σπάρτης) was a title instituted in 1868 to designate the Crown Prince of the Kingdom of Greece. Its legal status was exceptional, as the Greek constitution forbade the award or acceptance of titles of nobility for Greek citizens. Consequently, it was mostly used abroad, although also in unofficial use within Greece.
In 1868, when the Crown Prince Constantine (later king as Constantine I) was born, King George I issued a decree according to which Constantine, as well as any future heir to the Greek crown, would bear the title "Duke of Sparta". However, this decree was contrary to the Greek constitution, since nobility titles had never existed in Greece and the Greek constitution expressly outlawed them.
This led to a stormy debate in Parliament, but the government of the day backed the King on an argument that the constitutional provision did not apply to members of the Royal Family (even though its text made no distinction).
The decree was finally approved by Parliament, but use of the title "Duke of Sparta" within Greece was later quietly dropped. However, Crown Prince Constantine was known as "HRH The Duke of Sparta" on the international scene from his birth until his accession in 1913 - for 45 years.
This again led to the misunderstanding of various, quite respectable publications that the title "Duke of Sparta" was synonymous with that of "Crown Prince of Greece", and the title has thus re-surfaced from time to time, but neither of the successive Crown Princes of Greece have ever been officially styled thus. The term (literally, "heir"), which does not have any connotations of a nobility title, has been historically employed to denote the position of Crown Prince in general, not limited to the Greek throne, instead.