Cleomenes I (/kliːˈɒmɪniːz/; Greek Κλεομένης; died c. 490 BC) was Agiad King of Sparta from c. 524 to c. 490 BC. One of the most important Spartan kings, Cleomenes was instrumental in organising the Greek resistance against the Persian Empire of Darius, as well as shaping the geopolitical balance of Classical Greece.

Most of the life of Cleomenes is known through the Histories of Herodotus, an Athenian historian of the second half of the 5th century. He is one the most important characters of books 5 and 6, covering the decades before the Persian Wars. Herodotus's account however contains many mistakes, especially on the chronology of several major events, and is also very biased against Cleomenes. It seems that Herodotus got his information on Cleomenes from his opponents: the descendants of his half-brothers Leonidas and Cleombrotus, as well as those of Demaratus, the other Spartan king who was deposed by Cleomenes in 491. Herodotus for instance states that Cleomenes's reign was short; however he ruled for about 30 years. Demaratus conversely receives positive treatment in the Histories, even though he betrayed the Greeks to the Persians during the First Invasion of Greece. Paul Cartledge writes that Cleomenes suffered from a damnatio memoriae from the Spartans, notably for having corrupted the Oracle of Delphi in 491.

Other historians, however, identify aspects of the Herodotean account which do not conform with the negative portrayal of Cleomenes by his opponents, demonstrating that his account was not totally influenced by them. The mistake of Cleomenes's reign has often been identified as a fault of the text, rather than of Herodotus: Wilson's 2015 edition of the text therefore amends the passage by adding ἔτι, meaning still or yet, so that it reads "Cleomenes did not reign for much longer". Elsewhere, Cleomenes dutifully reports to the ephors about an attempt to bribe him, and, when campaigning at Eleusis, it is Demaratus who unfaithfully deserts him. Herodotus even explicitly states at one point that, unlike Demeratus, Cleomenes was "working for the common good of Greece".

Cleomenes was the son of Anaxandridas II, who belonged to the Agiad dynasty, one of the two royal families of Sparta (the other being the Eurypontids). As his father did not have a son from his first wife (who was also his niece), the ephors forced him to marry another woman, without divorcing his first wife—an unprecedented occurrence of bigamy in Sparta. His new spouse likely came from the family of the ephor Chilon, an important reformer, who held office in during the mid-6th-century. Cleomenes was born from this second marriage, but then his father returned to his first wife and had three further sons with her: Dorieus, the future king Leonidas, and Cleombrotus—the latter two were possibly twins. The name Dorieus ("the Dorian") explicitly refers to the Dorian ethnicity of Sparta, and might be a rejection of the ephor Chilon's policy of establishing an amicable relationship with the ethnically different Achaea in the northern Peloponnese.

The family of Anaxandridas's second wife immediately contested the legitimacy of Dorieus even before his birth, as the ephors attended his birth in order to certify the authenticity of the pregnancy. This shows that there were strong familial rivalries among Spartan royal circles; besides, at the same time, a cousin of Anaxandridas's second wife was also the bride of the future Eurypontid king Leotychidas. In turn, when his father died, Cleomenes's succession was contested by Dorieus, because of his superior "manly virtue". Perhaps this statement is related to a great performance during the agoge—the rigorous military training at Sparta—which Dorieus had to endure, while Cleomenes avoided it as heir-apparent (the only possible exemption). Dorieus could have also contested Cleomenes's legitimacy on the ground that he was a son of the king's first wife, who was additionally of royal descent. As Cleomenes was the eldest son, his claim was nevertheless deemed stronger and he became king. It prompted the departure of Dorieus to colonial ventures in Libya and Sicily, where he died in c.510.

The date of Cleomenes's accession had been debated among modern scholars for a long time, until historian David Harvey found that the Greek historian Diodoros of Sicily had confused the length of Cleomenes II's reign (370–309) with that of his earlier namesake. Putting aside Diodoros's error, Harvey states that as Cleomenes came to the throne "a few years earlier than the Plataea incident", he dates the start of his reign to 524–523.

During the first years of his reign, Cleomenes adopted prudent diplomacy, rejecting foreign expeditions when solicited, possibly due to the threat of a helot revolt that a defeat in a war abroad would cause.

The first known deed of Cleomenes as king is his dealing with the city of Plataea, located between Thebes and Athens. In 519, Herodotus states that Cleomenes happened to be in the vicinity of Plataea, when the Plataeans requested an alliance with Sparta, which he rejected. Instead he advised them to ally themselves with Athens, because he wanted to stir a border conflict between Thebes and Athens, two of the most powerful poleis of central Greece. The Plataeans probably wished to avoid their forced incorporation into the Boeotian League, which was being built by Thebes at this time. Their Spartan alliance request perhaps indicates that they wanted to become a member of the Peloponnesian League, which was likewise being put in place at this time. G. E. M. de Ste. Croix and Paul Cartledge call this move "a master-stroke" of diplomacy, but other modern scholars do not believe it was Cleomenes's intention to create a rift between Thebes and Athens.