Casio CA53W-1 Calculator - Finn Wolfhard - Stranger Things
In the first seasons he wears a calculator watch as well, but not the classic Casio. But based on one of the first promotional images of Season 3, it looks like he finally wears a real Casio CA53W-1 Calculator. We can just make out the shape of the case, the Casio logo and the dark colored buttons.
The Casio CA53W-1 Calculator watch was made famous in the 1980s by Marty McFly in the Back to the Future movie and is also worn by The Joker in The Dark Knight and Walther White in Breaking Bad.