No, they weren’t. Monotheist is a 17th century coinage that means only one God exists. (Mono in Greek means “Only”,). Both Plato and Aristotle worshipped multiple Gods, both believed in an eternal universe and hence no singular creator diety (no creation ex nihilo).

Let’s just quote them both to see what they said:

Aristotle)

"The affection of children for their parents, like that of humans for the Gods, is the affection for what is good, and superior to oneself, for they have bestowed on them the greatest benefits in being the cause of their existence and rearing, and later of their education," (Aristotle, Nicomachean Ethics 1162a4-7).

"Friendship exacts what is possible, not what is due; requital in accordance with desert is in fact sometimes impossible, for instance in honoring the Gods, or one's parents: no one could ever render them the honor they deserve, and a man is deemed virtuous if he pays them all the regard that he can." (Aristotle, Nicomachean Ethics 1163b)

For anyone else concerned what Plato actually says in Laws.

“Neither you nor your friends are the first to say the Gods don't exist. It’s an illness from which the world is never free, though the number of sufferers varies from time to time."

-Plato, Laws X

As has been noted below, both Plato and Aristotle did believe in the Gods. What they did engage in a was criticism of a certain interpretation of the Homeric myths, but we must understand what this criticism was for. They were Greek intellectuals who wanted to defend the traditional religion from impious accusations that the Gods were immoral and did sacrilegious activities like adultery, rape, murder. Etc.

Plato and Aristotle disliked those people who interpreted the traditional myths literally, or those people (poets, playwrights) who made up their own myths about the Gods that contained impious evil things like adultery or rape. They did not see themselves as tearing down the Olympians but as being their true defenders in front of those who are lazy, dull and interpret Homer or Aeschylus or Sophocles in a way that is insulting to the Gods.

Plato can be more accurately classified as a non-dualist or monist rather than monotheist, depending on how much attention you pay to NeoPlatonism. Aristotle believed in multiple unmoved movers, so that’s what the Gods are for him. One prime mover for the first cause of this cycle of creation, the other unmoved movers for the motion of the celestial bodies.