Answer: At zero carbs and nearly all fat, butter is a very keto food.
The keto diet is all about fats. Butter is almost nothing but fat. It's a match made in heaven!
Butter is zero carbs and is a good source of fats, at 11-12 g of fats per tablespoon. Some keto enthusiasts even add a tablespoon or two of butter to their coffee to bump up their fat counts for the day.
Butter is available in both salted and unsalted varieties. With regard to keto, the difference is irrelevant. Go with whichever you prefer.
One issue some keto eaters run into with butter is that, as a dairy product, it contains some proteins that give people digestive issues, namely lactose, casein, and whey. If you fall into this category, some keto-friendly butter alternatives are ghee and coconut oil.