How do you say you're dead in Japanese?

This translation depends quite largely on what is meant by "you're dead".If you literally wanted to inform someone that they were dead (had already died), you would say either あなたが死んでいます (anata ga shindeimasu) or あなたが死んじゃったんです (anata ga shinjattan desu).

To try to intimidate someone by saying "you're dead" (I will kill you), one would say お前殺すぞ (omae korosu zo).