The name Telendos of this small island of the Dodecanese, indicates an area (-indos) at the end of a long strip of land (-tel-, end, beyond). Indeed, the settlement of the island has exactly this image. Possibly Telendos is the ancient "Keleris". However, the name Telendos is considered to be of pre-Greek origin by the newest researchers.
In the maps of travelers and geographers of the Late Middle Ages it is sometimes stated under the name "Agalia" or "Klarus" and sometimes it is not drawn at all. In the Map of Riga Feraeus it is mentioned as "Klaros".
Around 1420 AD the Florentine monk and scholar Christoforo Buondelmonti is hosted in Rhodes by the Knights of St John in order to map the islands of the south-eastern Aegean, who refers to the island by the name of Telendos, describes it as uninhabited, informs us that goats and sheep live there and describes its history monuments.
Also, in a handwritten Greek portolan of the beginning of the 16th century, it is mentioned that opposite the old castle of Gorgona (today's Kastelli), on the western side of Kalymnos, there is a high island called Telendos.
In 1688 the geographer Francesco Piazenza who maps the Aegean gives us the information about the existence of a pine forest in Telendo and seems to imply that the island was uninhabited at that time.
The historical course of Telendos is intertwined with that of Kalymnos.
From rescue excavations carried out on the island, it is concluded that the area where the settlement is built today was inhabited during the Late Hellenistic period, expanded during the Roman period and reached its peak in the Early Christian years. The theater remains are the most important of the few pre-Christian monuments.
Between the 5th and 7th c. A.D. Telendos is considered an important Christian center next to Kalymnos. Today there are ruins of five early Christian basilicas, inside and mainly on the outskirts of the settlement.
In the year 551 or 553 an earthquake hit Kos and Kalymnos, which was even described by Agathias Scholastics.
This earthquake caused extensive damage to the basilicas, the baths and the buildings of the settlement, as well as the sinking of many buildings along the eastern beach of the island. Subsequently some buildings and temples were rebuilt.
In the middle of the 7th century the settlement was abandoned and the inhabitants moved further north on the island and at an altitude of about 300 m. They built their houses with tanks which are preserved to this day in good condition, which is due to the excellent quality of the hydraulic mortar with which they have smeared internally. The settlement was fortified with a wall on the sea side and two gates were built, the western one having a tower for protection as well as a double row of walls.
In the 11th century, the Telendians decided to return near the sea and at that time they rebuilt part of the basilica of Saint Basil. The lifespan of the Middle Byzantine settlement is impossible to determine. It was probably short, as no mid-Byzantine or late-Byzantine churches were built in Telendo.
Then the inhabitants moved to Kalymnos and the small island was deserted for centuries. At the beginning of the 20th century, spongers, fishermen and sailors settled on the eastern side of Telendos.

During the short rescue excavations that have been carried out in Telendos, mainly architectural remains of various periods were found which record in very rough lines the history of the island.
The area where the settlement is built today was inhabited during the Late Hellenistic period, expanded during the Roman period and reached its peak in the Early Christian years.
Remains of historic fortifications were found in the north, as well as a rock-hewn theater, which is the most important of the early Christian monuments. Also, in the place where the current settlement is located, there are ruins from Roman and Byzantine times.
Between the 5th and 7th c. AD Telendos flourished and from that period remains of three Paleo-Christian period thermal complexes, remains of an Paleo-Christian settlement, as well as architectural remains of three three-aisled Paleo-Christian basilicas have been found. An early Christian necropolis was also investigated at Pefki or Tholaria.