The Daylamites or Dailamites (Middle Persian: Daylamīgān; Persian: دیلمیان Deylamiyān) were an Iranian people inhabiting the Daylam—the mountainous regions of northern Iran on the southwest coast of the Caspian Sea, now comprising the southeastern half of Gilan Province.

The Daylamites were warlike people skilled in close combat. They were employed as soldiers during the Sasanian Empire and in the subsequent Muslim empires. Daylam and Gilan were the only regions to successfully resist the Muslim conquest of Persia, although many Daylamite soldiers abroad accepted Islam. In the 9th century many Daylamites adopted Zaidi Islam. In the 10th century some adopted Isma'ilism, then in the 11th century Fatimid Isma'ilism and subsequently Nizari Isma'ilism. Both the Zaidis and the Nizaris maintained a strong presence in Iran up until the 16th century rise of the Safavids who espoused the Twelver sect of Shia Islam. In the 930s, the Daylamite Buyid dynasty emerged and managed to gain control over much of modern-day Iran, which it held until the coming of the Seljuk Turks in the mid-11th century.

The Daylamites lived in the highlands of Daylam, part of the Alborz range, between Tabaristan and Gilan.

They spoke the Daylami language, a now-extinct Northwestern Iranian language similar to that of the neighbouring Gilites. During the Sasanian Empire, they were employed as high-quality infantry. According to the Byzantine historians Procopius and Agathias, they were a warlike people and skilled in close combat, being armed each with a sword, a shield, and spears or javelins.

The Daylamites first appear in historical records in the late 2nd century BC, where they are mentioned by Polybius, who erroneously calls them "Elamites" (Ἐλυμαῖοι) instead of "Daylamites" (Δελυμαῖοι). In the Middle Persian prose Kar-Namag i Ardashir i Pabagan, the last ruler of the Parthian Empire, Artabanus V (r. 208–224) summoned all the troops from Ray, Damavand, Daylam, and Padishkhwargar to fight the newly established Sasanian Empire. According to the Letter of Tansar, during this period, Daylam, Gilan, and Ruyan belonged to the kingdom of Gushnasp, who was a Parthian vassal but later submitted to the first Sasanian emperor Ardashir I (r. 224–242).

The descendants of Gushnasp were still ruling until in ca. 520, when Kavadh I (r. 488–531) appointed his eldest son, Kawus, as the king of the former lands of the Gushnaspid dynasty. In 522, Kavadh I sent an army under a certain Buya (known as Boes in Byzantine sources) against Vakhtang I of Iberia. This Buya was a native of Daylam, which is proven by the fact that he bore the title wahriz, a Daylamite title also used by Khurrazad, the Daylamite military commander who conquered Yemen in 570 during the reign of Khosrow I (r. 531-579), and his Daylamite troops would later play a significant role in the conversion of Yemen to the nascent Islam. The 6th-century Byzantine historian Procopius described the Daylamites as;

The equipment of the Dailamites of the Sasanian army included swords, shield, battle-axe (tabar-zīn), slings, daggers, pikes, and two-pronged javelins (zhūpīn).

Daylamites also took part in the siege of Archaeopolis in 552. They supported the rebellion of Bahrām Chōbin against Khosrow II, but he later employed an elite detachment of 4000 Daylamites as part of his guard. They also distinguished themselves at the Yemeni campaign of Wahriz and in the battles against the forces of Justin II.