Deioces was the founder and the first king of the Median Kingdom, an ancient polity located in the Iranian plateau. His name has been mentioned in different forms in various sources, including the ancient Greek historian Herodotus.

The exact date of the era of Deioces' rule is not clear and probably covered most of the first half of the 7th century BC. According to Herodotus, Deioces was the first Median king to have gained independence from the Neo-Assyrian Empire and governed for 53 years. After Deioces' death, his son, Phraortes, succeeded him.

Deioces' name has been mentioned in various forms in different sources. The Greek historian Herodotus has stated his name as Δηϊόκης (Dēiokēs). In Assyrian texts, he has been mentioned as Da-a-a-uk-ku; and in Elamite ones, as Da-a-(hi-)(ú-)uk-ka and Da-a-ya-u(k)-ka.

Deioces' name is derived from the Iranian Dahyu-ka, and is the junior noun of the word dahyu-, meaning "the land". The old Iranian name Deioces was not uncommon even in later times. In the Achaemenid period, the Old Persian form of Deioces has been mentioned in several Elamite inscriptions of the mud plates of Persepolis. Those mentions apparently referred to different persons in separate government regions; and one of them was an individual assigned to the food rationing of the horses.

Friedrich von Spiegel believes that Dahayuku means "resident and headman of the village" and is in fact the older form of the word dehghan "farmer". Also following Spiegel's theory, Ferdinand Justi believes that Deioces' name is his title and a shortened form of dahyaupati in Old Persian and danhupaiti in Avestan having acquired the suffix -ka. The name of Deioces may also be related to the modern Kurdish name Diyako.

The era of Deioces' reign is subject to controversy. Herodotus says that Deioces ruled for 53 years and thus some assumptions have been made about the era of his reign; but it seems that Herodotus's report is based on a verbal narrative. Based on Herodotus's report, the researchers have concluded that Deioces was the founder of the Median kingdom and also the first Median king having gained independence from the Assyrians. But Herodotus's report is a mixture of Greek and Oriental legends and is not historically reliable. Also, it is assumed that the Median king whom Herodotus's reports are about is the same Deioces, Phraortes' father; thus, it is not possible to clarify the exact date of the period of his rule; but it can be said that it probably covered most of the first half of the 7th century BC. Igor Diakonoff says: "The state of the era of Deioces' reign in Herodotus's writings is so different from the picture of that time (745-675 BC) described by the Assyrian sources that some historians have rejected Herodotus's statement."

In Assyrian sources, 674 BC, there are mentions of the actions of a person called Kashthrita, whom some researchers believe to be the same Phraortes. Therefore, the year 674 BC can be considered the end of Deioces' rule; and by reckoning his fifty-three-year old reign, the beginning of the era of Deioces' rule should be around 728 BC. Below is a list of the era of Deioces' reign based on the historians' views:

Herodotus's Deioces is sometimes associated with a provincial governor (šaknu) named Daiukku, mentioned several times in Neo-Assyrian texts from the time of Sargon II. Daiukku governed somewhat independently a district on the border between the Mannaeans and the Assyrian kingdoms. He allied with the king of Urartu against the Mannaen ruler but was captured by Sargon II, who exiled him and his family to Syria in 715 BC, where he apparently died. However, this association is highly unlikely, as the events mentioned would be located around Lake Urmia and not in Median territory. His namesake might have been one of the many chiefs of free Median districts outside Assyrian control. According to Herodotus, Deioces lived in the late 8th century BC, around the same time as the Median chief Daiku of Saparda. However, the identification would be more justifiable if Daiku were the chief of Hagabta, not Saparda. Hagbata is the only Median settlement mentioned in Assyrian sources whose name is similar to that of Ecbatana, the place chosen by Herodotus's Deioces to become the capital of the Median kingdom.