Is there any adjective that means satiety? When you feel hunger you say I'm hungry, what you say when you feel satiety? I know some people say "I'm full". But I wonder if there is a more formal alternative?
I believe you're looking for the term
He was hungry. Now he is satiated.
She ate a bit off all the plates in front of her until she was satiated.
You're probably less likely to hear this in casual conversation. I think of it as being almost more of a medical term, though others can step in if that's regional.
Merriam-Webster dictionary defines the noun form,
1: the quality or state of being fed or gratified to or beyond capacity : surfeit, fullness
2: the revulsion or disgust caused by overindulgence or excess
More generally, the opposite of feeling hungry would be being well-fed:
If you say that someone is well-fed, you mean that they get good food regularly.
I never felt hungry when I was growing up. I was always well-fed.