Theopompus was a Eurypontid king of Sparta

Kings of Sparta

Sparta was an important Greek city-state in the Peloponnesus. It was unusual among Greek city-states in that it maintained its kingship past the Archaic age. It was even more unusual in that it had two kings simultaneously, coming from two separate lines...

. He is believed to have reigned during the late 8th

8th century BC

The 8th century BC started the first day of 800 BC and ended the last day of 701 BC.-Overview:The 8th century BC was a period of great changes in civilizations. In Egypt, the 23rd and 24th dynasties led to rule from Nubia in the 25th Dynasty...

 and early 7th century BC.

Theopompus was the son and successor to Nicander.

The major event of his reign was the First Messenian War

Messenian Wars

Messenian Wars is a term of special historical application. It means the wars between Messenia and Sparta in the 8th and 7th centuries BC as well as the 4th century BC.*First Messenian War*Second Messenian War* Third Messenian War...

, which resulted in the defeat of the Messenia

Messenia

Messenia is a regional unit in the southwestern part of the Peloponnese region, one of 13 regions into which Greece has been divided by the Kallikratis plan, implemented 1 January 2011...

ns, for which, Pausanias

Pausanias (geographer)

Pausanias was a Greek traveler and geographer of the 2nd century AD, who lived in the times of Hadrian, Antoninus Pius and Marcus Aurelius. He is famous for his Description of Greece , a lengthy work that describes ancient Greece from firsthand observations, and is a crucial link between classical...

 reports, Tyrtaeus

Tyrtaeus

Tyrtaeus was a Greek poet who composed verses in Sparta around the time of the Second Messenian War, the date of which isn't clearly establishedsometime in the latter part of the seventh century BC...

 credits Theompompus:

To our king beloved of the gods, Theopompus, through whom we took Messene with wide dancing-grounds.

Pausanias reports that Theopompus was succeeded by his grandson Zeuxidamas or great-grandson Anaxidamus

Anaxidamus

Anaxidamus was a king of Sparta, 11th of the Eurypontids.He was the son of Zeuxidamus and contemporary with Anaxander, and lived to the conclusion of the Messenian Wars, 668 BC He was succeeded by his son Archidamus I.----...

, Theopompus' son Archidamus having predeceased him, though there is some evidence that his successor was Anaxandridas I, father of Zeuxidamus

Zeuxidamus

Zeuxidamus can refer to two ancient Spartans.#A king of Sparta, and tenth of the Eurypontid dynasty. He was grandson of Theopompus, son of Anaxandridas I, and father of Anaxidamus, who succeeded him....

.

According to Plutarch

Plutarch

Plutarch then named, on his becoming a Roman citizen, Lucius Mestrius Plutarchus , c. 46 – 120 AD, was a Greek historian, biographer, essayist, and Middle Platonist known primarily for his Parallel Lives and Moralia...

, it was in Theopompus' reign that the ephors were introduced in Sparta.