Bardiya or Smerdis (Old Persian: 𐎲𐎼𐎮𐎡𐎹 Bardiya; Ancient Greek: Σμέρδις Smérdis; possibly died 522 BCE), also named as Tanyoxarces (Old Persian: *Tanūvazraka; Ancient Greek: Τανυοξάρκης Tanuoxárkēs) by Ctesias, was a son of Cyrus the Great and the younger brother of Cambyses II, both Persian kings. There are sharply divided views on his life. Bardiya either ruled the Achaemenid Empire for a few months in 522 BCE, or was impersonated by a magus called Gaumata (Old Persian: 𐎥𐎢𐎶𐎠𐎫 Gaumāta), whose name is given by Ctesias as Sphendadates (Old Persian: *Spantadātah; Ancient Greek: Σφενδαδάτης Sphendadátēs), until he was toppled by Darius the Great.

The prince's name is listed variously in the historical sources. In Darius the Great's Behistun inscription, his Persian name is Bardiya or Bardia. Herodotus calls him Smerdis, which is the prevalent Greek form of his name; the Persian name has been assimilated to the Greek (Asiatic) name Smerdis or Smerdies, a name which also occurs in the poems of Alcaeus and Anacreon. Bardiya is called Tanyoxarces by Ctesias, who also names Gaumāta as Sphendadates; he is called Tanooxares by Xenophon, who takes the name from Ctesias, and he is called Mergis and Merdis by Justin and Merdis by Aeschylus.

In English-language histories he has traditionally been called Smerdis, following Herodotus' example, but recent histories tend to call him Bardiya.

The traditional view is based on several ancient sources, including the Behistun inscription as well as Herodotus, in Ctesias, and Justin, although there are minor differences among them. The three oldest surviving sources agree that Gaumata/Pseudo-Smerdis/Sphendadates was overthrown by Darius and others in a coup d'état, and that Darius then ascended the throne. Most sources (including Darius himself, Herodotus and Ctesias) have Darius as part of a group of seven conspirators. In Greek and Latin sources, Darius subsequently gained kingship by cheating in a contest.

Bardiya was the younger son of Cyrus the Great and a full or half-brother of Cambyses II. According to Ctesias, on his deathbed Cyrus appointed Bardiya as satrap (governor) of some of the far-eastern provinces. According to Darius the Great, Cambyses II, after becoming king of Persia but before setting out for Egypt, killed Bardiya and kept this secret. However, according to Herodotus (who gives two detailed stories), Bardiya went to Egypt with Cambyses and was there for some time but later Cambyses sent him back to Susa out of envy, because "Bardiya alone could draw the bow brought from the Ethiopian king." Herodotus then states that "Cambyses had a dream in which he saw his brother sitting on the royal throne. As a result of this dream Cambyses sent his trusted counselor Prexaspes from Egypt to Susa with the order to kill Smerdis" (i.e., Bardiya).

Bardiya's death was not known to the people, and so in the spring of 522 BC, a usurper pretended to be him and proclaimed himself king on a mountain near the Persian town of Paishiyauvada. Darius claimed that the real name of the usurper was Gaumata, a Magian priest from Media; this name has been preserved by Justin but given to his brother (called Patizeithes by Herodotus), who is said to have been the real promoter of the intrigue. According to Herodotus, the name of the Magian usurper was Oropastes, but according to Ctesias it was Sphendadates.^{[citation needed]}

The despotic rule of Cambyses, coupled with his long absence in Egypt, contributed to the fact that "the whole people, Persians, Medes and all the other nations," acknowledged the usurper, especially as he granted a tax relief for three years. Cambyses began to march against him, but died in the spring of 522 BC in disputed circumstances. Before his death he confessed to the murder of his brother, and publicly explained the whole fraud, but this was not generally believed. Nobody had the courage to oppose the new king, who ruled for seven months over the whole empire. The new king transferred the seat of government to Media. A number of Persian nobles discovered that their new ruler was an impostor, and a group of seven nobles formed a plot to kill him. In September 522 BC they surprised him at a castle in Nisa (on the Nisaean plain) and stabbed him to death. One of the seven, Darius, was proclaimed as ruler shortly after.^{[citation needed]}

While the primary sources do not agree on the names and many other details, the three oldest surviving sources (Darius himself, Herodotus and Ctesias) all portray Gaumata/Pseudo-Smerdis/Sphendadates as an imposter who usurped the throne by posing as one of the sons of Cyrus the Great, i.e. as one of the brothers of Cambyses II. In Darius' trilingual Behistun inscription, the prince being impersonated is named "Pirtiya" in Elamite, "Bardiya" in Old Persian, and "Barziya" in Akkadian. In Herodotus' Histories, the prince and his imposter have the same name (Smerdis). For Ctesias, Sphendadates poses as 'Tanyoxarces'. Other Greek sources have various other names for the figure being impersonated, including 'Tanoxares', 'Mergis' and 'Mardos'.