You do not have to refrigerate butter.
Butter is safe to leave at room temperature but refrigerating butter or even freezing butter helps extend the shelf life of the butter.
However I've left butter at room temperature for quite a while now and it's never been an issue.
We use a lot of butter in our house so I like to keep the butter at room temperature so that it's easy to spread without having to wait for the butter to warm up.
According to the USDA, butter is safe at room temperature.
But if it's left out for several days at room temperature, it can turn rancid causing off flavors.
The USDA does not recommend leaving it out more than one to two days.
Salted butter has less of a chance of spoiling on the counter compared to unsalted butter.
Left unwrapped, either salted or unsalted butter will keep in the refrigerator for at least 8 weeks.
Salted butter, in fact, will last an additional 4 weeks, since salt acts as a preservative.
Once you open the wrap, however, butter should be used with in 3 weeks.
Butter is about 80 percent fat, which makes its water content much lower and therefore less likely to have bacteria when left out.
The salt in your butter also wards off bacteria growth—but, if you have unsalted butter, it's best kept in the fridge.