Here's what it says on their site:
Originally Posted by Thermoworks
While a few of our products are manufactured here, many are made for us by partner factories in Europe, Asia and the USA.
You could call them up and find out for sure.
In any case, quality depends on the parts and the labor. Apple iPads are made in the same country as your worst crappy fake knockoff phones, after all.
Anyways, TW will back up their warranty, and that's what's important.