Upon his return to his family home, Angel has two letters waiting for him: Tess's angry note and a few cryptic lines from "two well-wishers" (Izz and Marian), warning him to protect his wife from "an enemy in the shape of a friend". He sets out to find Tess and eventually locates Joan, now well-dressed and living in a pleasant cottage. After responding evasively to his enquiries, she tells him Tess has gone to live in Sandbourne, a fashionable seaside resort. There, he finds Tess living in an expensive boarding house under the name "Mrs. d'Urberville." When he asks for her, she appears in startlingly elegant attire and stands aloof. He tenderly asks her forgiveness, but Tess, in anguish, tells him he has come too late; thinking he would never return, she yielded at last to Alec d'Urberville's persuasion and has become his mistress. She gently asks Angel to leave and never come back. He departs, and Tess returns to her bedroom, where she falls to her knees and begins a lamentation. She blames Alec for causing her to lose Angel's love a second time, accusing Alec of having lied when he said that Angel would never return to her.
The landlady, Mrs. Brooks, tries to listen in at the keyhole, but withdraws hastily when the argument becomes heated. She later sees Tess leave the house, then notices a spreading red spot – a bloodstain – on the ceiling. She summons help, and Alec is found stabbed to death in his bed.
Angel, totally disheartened, has left Sandbourne; Tess hurries after him and tells him that she has killed Alec, saying that she hopes she has won his forgiveness by murdering the man who ruined both their lives. Angel doesn't believe her at first, but grants his forgiveness and tells her that he loves her. Rather than head for the coast, they walk inland, vaguely planning to hide somewhere until the search for Tess is ended and they can escape abroad from a port. They find an empty mansion and stay there for five days in blissful happiness, until their presence is discovered one day by the cleaning woman.
They continue walking and, in the middle of the night, stumble upon Stonehenge, where Tess lies down to rest on an ancient altar. Before she falls asleep, she asks Angel to look after her younger sister, Liza-Lu, saying that she hopes Angel will marry her after she is dead. At dawn, Angel sees that they are surrounded by police. He finally realises that Tess really has committed murder and asks the men in a whisper to let her awaken naturally before they arrest her. When she opens her eyes and sees the police, she tells Angel she is "almost glad" because "now I shall not live for you to despise me". Her parting words are, "I am ready."
Tess is escorted to Wintoncester (Winchester) prison. The novel closes with Angel and Liza-Lu watching from a nearby hill as the black flag signalling Tess's execution is raised over the prison. Angel and Liza-Lu then join hands and go on their way.