Rusalka waits by the lake once again, lamenting her fate. Ježibaba appears and mocks her, then hands her a knife and explains that there is a way to save herself: she must kill the Prince. Rusalka refuses, throwing the weapon into the water. When her sisters reject her as well, she sinks into the lake in despair. The Gamekeeper and the Kitchen Boy arrive to ask Ježibaba for help. The Prince, they say, has been bewitched by a strange wood girl he was going to marry. Enraged, the Water Sprite rises from the lake, saying that it was the Prince who deceived Rusalka. Terrified by the supernatural sight, the two run away. The wood nymphs enter, singing and dancing, but when the Water Gnome explains to them what has happened to Rusalka, they fall silent and disappear.
The Prince, desperate and half crazy with remorse, emerges from the forest, looking for Rusalka and calling out for her to return to him. She appears from the water, reproaching him for his infidelity, and explains that now a kiss from her would kill him. Accepting his destiny, he asks her to kiss him to give him peace. She does, and he dies in her arms. Rusalka asks for mercy on his soul and disappears into the water.