Cape Sideros or Cape Sidero (Greek: Άκρα Σίδερος, romanized: Akra Sideros) is a cape at the eastern end of the island of Crete, Greece. Anciently it was known as Samonium or Samonion (Ancient Greek: Σαμώνιον), Sammonium or Sammonion (Ancient Greek: Σαμμώνιον), Salmonium or Salmonion (Σαλμώνιον) and Salmone (Σαλμώνη). The cape shares the name Sideros or Sidero with the island-like peninsula of which it is a projection, but which had the name first remains unknown, as does the provenance of either name. Cape Sidero is often not confined to the peninsula Sideros, but might refer to the entire northeast promontory.

The meaning of Sidero seems transparent at first glance, as the modern Greek meaning of sidero with a short e is "ferruginous." The ancient Greek word has a long e, but the shortening of the e is no linguistic obstacle to common descent. There is no evidence of the sense. What about the island or the cape is "iron" remains unknown. Iron is not in its mineral structure. There is a second word sidero, "sidereal," but no evidence exists of a connection to stars, either.

Another seemingly transparent connection is that sidero somehow has the same meaning as Samonium. As the latter word has no Greek translation, it would probably be non-Greek. There was substantial ancient Minoan colonization of the promontory. Its ancient and modern name, Itano or Itanos is the same as the name of the Greek city in the area; however, that name appears in Linear B as the name of a Minoan settlement, Utana, which suggests that Sidero and Samonium may not have been Greek either. Assigning the closest word in one language to a word in another is a common theme in the renaming of places by different cultures.

Forbes and Spratt, 19th century travellers over Ottoman Crete, offer the derivation Eis ten Etera > Sitera > Sidero. The starting point is "next to Etera." The latter city is an MSS or other-author variant of Itanos in the anonymous Stadiasmus Maris Magni, an ancient periplus ("sail around," a list of coastal ports, here on the shores of the mare magnum, the Mediterranean) published by Karl Müller in Geographi Graeci Minores. The other variants are Istros or Istron, Istronas or Istrona, Ittone, called by some Arsinoe. At that early date they could only conclude that an unknown city, Etera, lay above Eremoupolis Beach, where Eremoupolis means "deserted city." It was up to the archaeologists to discover later that the hills around the beach for some distance were the site of the sprawling port of Itanos with two acropoleis and two major churches, and that its name at abandonment had been Itanos and not Etera or Istria. Since the city was at its floruit as a Christian city at the time of the Periplus, and there was no room for any other city, one can only conclude that the author of the Periplus was an armchair geographer, like all the rest, without access to eastern Crete, which any ordinary person today can get via GPS and digital photography.

In another speculation, Sidero comes from Isidore, the name of the saint to whom the nearby church is dedicated. The linguistic gap of that derivation is somewhat harder to bridge. If both the cape and the church are named after the same saint and both are the same property, the development of such a difference in names requires an explanation.

The first use of Sidero for the island or the cape remains unknown. Modern dictionaries reflect a consensual belief that Cape Sidero and Cape Samonium always have been one and the same. Some dictionaries, however, report an original issue whether they were the same. Nothing considered predominantly certain remains yet hypothesized.

In the original disagreement, the alternative point of view that seemed to present itself to cartographers is that Samonium is what is called today Cape Plaka, although no cartographer apparently had any detailed knowledge of that cape when he designed his map. There was no Cape called Sidero. All cartographers worked from other maps, some having little or no independent information about the country they were mapping. The most influential map-maker of the late Roman Empire was Claudius Ptolemy. He had developed his own coordinate system, the first known surviving. The centuries did not preserve early maps, however, perhaps because they were in such demand. All of Ptolemy's maps were lost, but his reputation persisted. His Geography survived as a gazeteer of places with coordinates. It could not be used today, of course, as its distortions are numerous and great.

At the start of the Age of Exploration, the explorers, having not yet explored, and having no comprehensive maps, turned to the maps of their ancestors. The cartographers were happy to oblige with reconstructions. The issue of Cape Sidero comes from alternative reconstructions of Ptolemy. As these became more and more realistic, Ptolemy eventually went by the board.