Swatch Yellow Racer - Sadie Sink - Stranger Things
The watch has a yellow strap, yellow case, and a red / green / white / black checkered dial.
This Swatch watch is from the Originals series, from the year 1984. It came in both a Gents model (GJ400) and Ladies version (LJ100): the GJ400 is bigger and has a small date window at the 3 o'clock position, the LJ100 doesn't have the date window. Sadie Sink is wearing the Gents GJ400 version, as the date window at 3 o'clock position can just be spotted.
You can still find both the Swatch Yellow Racer GJ400 and LJ100 on eBay for anywhere between $35 - $180 depending on the condition of the watch.
Sadie Sink plays Maxine "Max" Mayfield in the popular Netflix series. Her character was introduced in Season 2. In Season 3 she can be seen once again and she wears the same yellow Swatch watch.