Crates (Ancient Greek: Κράτης ὁ Θηβαῖος; c. 365 – c. 285 BC) of Thebes was a Greek Cynic philosopher, the principal pupil of Diogenes of Sinope and the husband of Hipparchia of Maroneia who lived in the same manner as him. Crates gave away his money to live a life of poverty on the streets of Athens. Respected by the people of Athens, he is remembered for being the teacher of Zeno of Citium, the founder of Stoicism. Various fragments of Crates' teachings survive, including his description of the ideal Cynic state.

Crates was born c. 365 BC in Thebes. He was the son of Ascondus, and was the heir to a large fortune, which he is said to have renounced to live a life of Cynic poverty in Athens. Diogenes Laërtius preserves several different accounts of this story; one of them has Crates giving his money away to the citizens of Thebes, apparently after seeing the beggar king Telephus in a tragedy; whereas another account has him placing his money in the hands of a banker, with the agreement that he should deliver it to his sons, unless they too became philosophers, in which case he should distribute it among the poor.

He moved to Athens where tradition says he became a pupil of Diogenes of Sinope; the precise relationship between Crates and Diogenes is uncertain, but there is one apparent reference to Crates referring to himself as "a fellow-citizen of Diogenes, who defied all the plots of envy". Crates is also described as being the student of Bryson the Achaean, and of Stilpo. He lived a life of cheerful simplicity, and Plutarch, who wrote a detailed biography of Crates which does not survive, records what sort of man Crates was:

But Crates with only his wallet and tattered cloak laughed out his life jocosely, as if he had been always at a festival.

He is said to have been deformed with a lame leg and hunched shoulders. He was nicknamed the Door-Opener (Greek: θυρεπανοίκτης) because he would enter any house and people would receive him gladly and with honour:

He used to enter the houses of his friends, without being invited or otherwise called, in order to reconcile members of a family, even if it was apparent that they were deeply at odds. He would not reprove them harshly, but in a soothing way, in a manner which was non-accusatory towards those whom he was correcting, because he wished to be of service to them as well as to those who were just listening.

He attracted the attentions of Hipparchia of Maroneia, the sister of one of Crates' students, Metrocles. Hipparchia is said to have fallen in love with Crates and with his life and teachings, and thus rejecting her wealthy upbringing in a manner similar to Crates, she married him. The marriage was remarkable (for ancient Athens) for being based on mutual respect and equality between the couple. Stories about Hipparchia appearing in public everywhere with Crates are mentioned precisely because respectable women did not behave in that way, and as part of Cynic shamelessness, they had sexual intercourse in public. They had at least two children, a girl, and a boy named Pasicles. We learn that Crates is supposed to have initiated his son into sex by taking him to a brothel, and he allowed his daughter a month's trial marriage to potential suitors.

He was the teacher of Zeno of Citium in the last years of the century, and was undoubtedly the biggest influence on Zeno in his development of Stoic philosophy. Zeno always regarded Crates with the greatest respect, and some of the accounts we have of Crates have probably come down to us via Zeno's writings. His other pupils included Metrocles, Monimus, Menippus, Cleomenes, Theombrotus, and Crates' brother Pasicles. He may also have taught Cleanthes, Zeno's successor as head of the Stoic school.