Siddhartha, the handsome and respected son of a Brahmin, lives with his father in ancient India. Everyone in the village expects Siddhartha to be a successful Brahmin like his father. Siddhartha enjoys a near-idyllic existence with his best friend, Govinda, but he is secretly dissatisfied. He performs all the rituals of religion, and he does what religion says should bring him happiness and peace. Nonetheless, he feels something is missing. His father and the other elders have still not achieved enlightenment, and he feels that staying with them will not settle the questions he has about the nature of his existence. Siddhartha believes his father has already passed on all the wisdom their community has to offer, but he longs for something more.

One day, a group of wandering ascetics called Samanas passes through town. They are starved and almost naked and have come to beg for food. They believe enlightenment can be reached through asceticism, a rejection of the body and physical desire. The path the Samanas preach is quite different from the one Siddhartha has been taught, and he believes it may provide some of the answers he is looking for. He decides to follow this new path. Siddhartha’s father does not want him to join the Samanas, but he cannot dissuade Siddhartha. Govinda also wants to find a path to enlightenment, and he joins Siddhartha in this new life.

Siddhartha adjusts quickly to the ways of the Samanas because of the patience and discipline he learned in the Brahmin tradition. He learns how to free himself from the traditional trappings of life, and so loses his desire for property, clothing, sexuality, and all sustenance except that required to live. His goal is to find enlightenment by eliminating his Self, and he successfully renounces the pleasures of the world.

Sunburned and half-starved, Siddhartha soon ceases to resemble the boy he used to be. Govinda is quick to praise the Samanas and notes the considerable moral and spiritual improvements they both have achieved since joining. Siddhartha, however, is still dissatisfied. The path of self-denial does not provide a permanent solution for him. He points out that the oldest Samanas have lived the life for many years but have yet to attain true spiritual enlightenment. The Samanas have been as unsuccessful as the Brahmins Siddhartha and Govinda left behind. At this time, Siddhartha and the other Samanas begin to hear about a new holy man named Gotama the Buddha who has attained the total spiritual enlightenment called Nirvana. Govinda convinces Siddhartha they both should leave the Samanas and seek out Gotama. Siddhartha and Govinda inform the leader of the Samanas of their decision to leave. The leader is clearly displeased, but Siddhartha silences him with an almost magical, hypnotizing gaze.

Siddhartha and Govinda find Gotama’s camp of followers and are taken in. Siddhartha is initially pleased with Gotama, and he and Govinda are instructed in the Eightfold Path, the four main points, and other aspects of Buddhism. However, while Govinda is convinced to join Gotama and his followers, Siddhartha still has doubts. He has noticed a contradiction in Gotama’s teachings: Siddhartha questions how one can embrace the unity of all things, as the Buddha asks, if they are also being told to overcome the physical world. Siddhartha realizes Buddhism will not give him the answers he needs. Sadly, he leaves Govinda behind and begins a search for the meaning of life, the achievement of which he feels will not be dependent on religious instruction.

Siddhartha decides to embark on a life free from meditation and the spiritual quests he has been pursuing, and to instead learn from the pleasures of the body and the material world. In his new wanderings, Siddhartha meets a friendly ferryman, fully content with his simple life. Siddhartha crosses the ferryman’s river and comes to a city. Here, a beautiful courtesan named Kamala entrances him. He knows she would be the best one to teach him about the world of love, but Kamala will not have him unless he proves he can fit into the material world. She convinces him to take up the path of the merchant. With her help, Siddhartha soon finds employment with a merchant named Kamaswami and begins to learn the trade. While Siddhartha learns the wisdom of the business world and begins to master the skills Kamaswami teaches him, Kamala becomes his lover and teaches him what she knows about love.

Years pass, and Siddhartha’s business acumen increases. Soon, he is a rich man and enjoys the benefits of an affluent life. He gambles, drinks, and dances, and anything that can be bought in the material world is his for the taking. Siddhartha is detached from this life, however, and he can never see it as more than a game. He doesn’t care if he wins or loses this game because it doesn’t touch his spirit in any lasting way. The more he obtains in the material world, the less it satisfies him, and he is soon caught in a cycle of unhappiness that he tries to escape by engaging in even more gambling, drinking, and sex. When he is at his most disillusioned, he dreams that Kamala’s rare songbird is dead in its cage. He understands that the material world is slowly killing him without providing him with the enlightenment for which he has been searching. One night, he resolves to leave it all behind and departs without notifying either Kamala or Kamaswami.

Sick at heart, Siddhartha wanders until he finds a river. He considers drowning himself, but he instead falls asleep on the riverbank. While he is sleeping, Govinda, who is now a Buddhist monk, passes by. Not recognizing Siddhartha, he watches over the sleeping man to protect him from snakes. Siddhartha immediately recognizes Govinda when he wakes up, but Govinda notes that Siddhartha has changed significantly from his days with the Samanas and now appears to be a rich man. Siddhartha responds that he is currently neither a Samana nor a rich man. Siddhartha wishes to become someone new. Govinda soon leaves to continue on his journey, and Siddhartha sits by the river and considers where his life has taken him.

Siddhartha seeks out the same content ferryman he met years before. The ferryman, who introduces himself as Vasudeva, radiates an inner peace that Siddhartha wishes to attain. Vasudeva says he himself has attained this sense of peace through many years of studying the river. Siddhartha expresses a desire to likewise learn from the river, and Vasudeva agrees to let Siddhartha live and work beside him. Siddhartha studies the river and begins to take from it a spiritual enlightenment unlike any he has ever known. While sitting by the river, he contemplates the unity of all life, and in the river’s voice he hears the word Om.

One day Kamala the courtesan approaches the ferry along with her son on a pilgrimage to visit Gotama, who is said to be dying. Before they can cross, a snake bites Kamala. Siddhartha and Vasudeva tend to Kamala, but the bite kills her. Before she dies, she tells Siddhartha that he is the father of her eleven-year-old son. Siddhartha does his best to console and provide for his son, but the boy is spoiled and cynical. Siddhartha’s son dislikes life with the two ferrymen and wishes to return to his familiar city and wealth. Vasudeva believes Siddhartha’s son should be allowed to leave if he wants to, but Siddhartha is not ready to let him go. One morning, Siddhartha awakens to find his son has run away and stolen all of his and Vasudeva’s money. Siddhartha chases after the boy, but as he reaches the city he realizes the chase is futile. Vasudeva follows Siddhartha and brings him back to their home by the river, instructing him to soothe the pain of losing his son by listening to the river.

Siddhartha studies the river for many years, and Vasudeva teaches Siddhartha how to learn the many secrets the river has to tell. In contemplating the river, Siddhartha has a revelation: Just as the water of the river flows into the ocean and is returned by rain, all forms of life are interconnected in a cycle without beginning or end. Birth and death are all part of a timeless unity. Life and death, joy and sorrow, good and evil are all parts of the whole and are necessary to understand the meaning of life. By the time Siddhartha has learned all the river’s lessons, Vasudeva announces that he is through with his life at the river. He retires into the forest, leaving Siddhartha to be the ferryman.

The novel ends with Govinda returning to the river to seek enlightenment by meeting with a wise man who lives there. When Govinda arrives, he does not recognize that the wise man is Siddhartha himself. Govinda is still a follower of Gotama but has yet to attain the kind of enlightenment that Siddhartha now radiates, and he asks Siddhartha to teach him what he knows. Siddhartha explains that neither he nor anyone can teach the wisdom to Govinda, because verbal explanations are limited and can never communicate the entirety of enlightenment. Instead, he asks Govinda to kiss him on the forehead, and when Govinda does, the vision of unity that Siddhartha has experienced is communicated instantly to Govinda. Govinda and Siddhartha have both finally achieved the enlightenment they set out to find in the days of their youth.