Stack Exchange network consists of 175 Q&A communities including Stack Overflow, the largest, most trusted online community for developers to learn, share their knowledge, and build their careers.
Here are two screenshots from Stranger Things S03E08 with the gadget in question:
It does not look like a wrist watch. Is it maybe some kind of portable video game?
This is a Nelsonic Q'Bert Wristwatch, sold by Nelsonic Digital. They were very popular, especially in the 80s for their game-themed wristwatches, wherein you could actually play the game on your wrist!