Plato's theory of knowledge does not identify knowledge with justified true belief. I've said it before, but I've recently come across several cases of people referring to the opposite as (for instance) "well-known"; to the extent that it is "well-known", it is one of those "well-known" things that is false and does not stand up to serious examination of evidence. The usual source identified, of course, is the Theaetetus. In this dialogue, Plato's Socrates examines three proposals for what knowledge is:
true judgment with logos
Logos is always a tricky word, but here it means something like 'account', 'explanation', 'reason'. Whether "true judgment with logos" can be prodded and poked into a shape equivalent to "justified true belief" is a difficult question, depending heavily on what assumptions one makes, but suppose it does. Nonetheless, in this dialogue Socrates rejects all three positions. The last one he even calls a bunch of wind before he heads out to his trial. The problem with it is that, under any plausible theory of what counts as logos, the definition either (a) makes knowledge indistinguishable from various kinds of lucky opinion; or (b) just means "true judgment with whatever it is that would turn true judgment into knowledge". The famous Gettier cases against JTB as an analysis of knowledge are really taking advantage of what Plato had already pointed out in (a). Even if one takes Plato to have considered JTB as an account of knowledge -- which is controvertible -- he rejected it as obviously untenable. At least, he has Socrates do so.
Plato's account of knowledge is not a JTB account. It needs to be repeated until professional philosophers stop saying that it is.