Yes. The theorem was attributed in antiquity to Pythagoras; in reality this probably means the Pythagoreans, since we have very little information about Pythagoras' own teachings, and his cult (or school) often gets conflated with him.

It's reasonably likely, though, that Pythagoras himself knew the truth of the theorem in the specific case of the 3-4-5 triangle; Plutarch reports that that special case was also known to "the Egyptians" (of what period, he does not say, and he may be referring to a time after Pythagoras; Plutarch On Isis and Osiris 373f-374a, ch. 56).

A few ancient sources report that Pythagoras was aware of the 3-4-5 case. For the general case, Euclid Elements book 1, proposition 47 gives the details of a proof, and Proclus' commentary on Euclid (old translation in [this PDF file](http://www.platonic-philosophy.org/files/Proclus%20-%20On%20Euclid's%20Elements%20(1%20of%202).pdf), p. 214 of the PDF = p. 204 according to the book's page numbering) attributes it to Pythagoras. Diogenes Laertios 8.12 also attributes knowledge of the general case to Pythagoras.

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Edit. After checking Burkert's Lore and Science in Ancient Pythagoreanism, I find him saying at p. 429 that

What Neugebauer first suggested as a possibility in 1928 has since then grown into a certainty -- namely, that the "Pythagorean theorem" had been used routinely for centuries in Babylon, and was therefore obviously not a discovery of the Greeks. It must have been introduced as a piece of Babylonian arithmetical technique. It is possible that Pythagoras was its intermediary; but, in view of the multiplicity of contacts between Greece and the Orient in the sixth century, the "fame" of Pythagoras can hardly be explained on this one ground. There is no testimony that he gave a strict proof of the theorem,^1 and this cannot be made to seem probable. The suspicion remains that the theorem had more than a mathematical significance in Pythagoras' school, and that the numbers involved seemed in a cryptic way meaningful. ...In fact, this fits especially well with the kind of number speculation we learn of from Aristotle, where 3 is male, 4 is female, and 5, which mysteriously unites them in the Pythagorean triangle, is "marriage."^2

^(1)rosemary85's note: that is, Burkert interprets Proclus not as assigning to Pythagoras the specific proof given by Euclid, but only as testifying to the tradition that the Pythagoreans had had a proof]

^(2)Burkert cites Plutarch On Isis and Osiris 56 here (see above), who reports on an "Egyptian" tradition that identified 3 as Osiris, 4 as Isis, and 5 as Horus.

The reasoning here is influenced by several factors: first, the mystical nature of the Pythagoreans (the Pythagoreans of the sixth century BCE were a cult, not a group of mathematicians, and their interest in numbers was more to do with gematria numerology than anything genuinely mathematical); second, the fact that the evidence for genuine mathematics being done in the Greek world only goes back to the fifth century BCE; and third, the fact that "Pythagoras' theorem" also pops up in ancient India and China.

I'm not so confident: I'm prepared to believe that the 3-4-5 special case was Babylonian in origin, and spread throughout Eurasia from there, but given how easy it is to come up with all sorts of proofs for the general case, I'm also prepared to believe that proofs of the general case were invented independently in India, China, and fifth-century BCE Greece. If so, it may not be wrong for the later Greco-Romans to attribute the theorem to the mathematikoi sect of the Pythagoreans (not to Pythagoras himself, of course, and not very likely to the akousmatikoi sect either).

Thank you