Buck is the hero of the story. He is a large, 140-pound dog, the offspring of a huge St. Bernard and a Scotch shepherd dog. At the beginning of the story Buck lives comfortably on a ranch with his owner, Judge Miller. Buck has a royal, dignified bearing; he is an aristocrat of a dog. However, when he is kidnapped and finds himself in the Arctic as part of a dog-team pulling a sled, he has to find new resources within himself in order to survive. He shows courage and cunning as he learns to master an environment in which the only law is the survival of the fittest. Buck also hears the call of the wild, which is the pull exerted by the primitive instincts that his new life awakens in him. Eventually he rejects the human world altogether and becomes the leader of a wolf pack.
Charles is a middle-aged man married to Mercedes. He and his brother-in-law Hal buy Buck and the other dogs cheaply in Skagway. Charles is from somewhere south and is out of place in the harsh Arctic environment. He and his companions overload the sled, and also miscalculate how much dog food is needed for their trip. He is perpetually wrangling with Hal. Charles drowns, with his companions and dogs, when the ice gives way as they try to make their way out of Thornton's camp in the spring.
Curly is a good-natured Newfoundland whom Buck meets at the outset of his trip north. Curly tries to make friends with another dog, but is attacked, and then torn to pieces by the pack.
Dave is a dog in Buck's team. He appears to take no interest in anything, but he is a good worker, and he is very stubborn. When he gets sick he would rather die than be taken out of harness. He eventually gets so weak he has to be shot.
Fran´┐Żois is the man who along with Perrault, takes charge of Buck early in the trip up north. He is a large man, of French-Canadian blood, and he knows how to enforce discipline amongst the dogs with his whip. But Fran´┐Żois, although he is necessarily harsh, is also fair-minded, and Buck learns to respect him.
Hal is the nineteen-year-old brother of Mercedes, and brother-in-law of Charles. He is inexperienced, foolish and quarrelsome. He needlessly whips and clubs the dogs because he thinks they are lazy. and shows little understanding of what is required in the Arctic environment. His refusal to take the advice of Thornton not to attempt to travel farther on the river because of melting ice costs him and all his party their lives.
Manuel is a gardener's helper on Judge Miller's ranch. He sells Buck in order to finance his gambling habit.
Mercedes is the sister of Hal and wife of Charles. She is ill-suited to traveling in the Arctic because she cannot leave behind the pampered life she has led. She adds to the difficulty of the trip, first by insisting that she packs a tent on the sled, and then when she insists on riding on the sled, rather than walking alongside it. In general, she acts like a spoiled child, although in her favor is the fact that she has sympathy for the dogs and tries to prevent them being ill-treated by Hal.
Judge Miller is Buck's owner for the first four years of the dog's life. He owns an estate in the Santa Clara Valley, in California, and he treats Buck well.
Perrault is a French-Canadian courier for the Canadian government. He is knowledgeable about dogs, and knows he is getting a bargain when he buys Buck for the trip north. Like his colleague Fran´┐Żois, Perrault is a fair man who administers justice to the dogs impartially. He is also extremely resourceful and is prepared to take risks on difficult journeys.
Sol-leks is a member of the dog-team. He is blind in one eye, and his name means the Angry One.
Spitz is Buck's main rival. He is the fierce, ruthless, amoral leader of the dog-team when Buck first arrives, and he is jealous of Buck. Their quarrel continues until it breaks out in a fight to the death, which Buck wins.
John Thornton is a gold hunter who rescues Buck when the dog is at death's door, having been mistreated by Hal. Thornton is an experienced man who knows how to live in the north. He is also kind to Buck, which is why Buck loves him and gives him complete loyalty. Just as Thornton saved Buck's life, Buck saves Thornton from drowning. Thornton is at home in the wild, and is not afraid of anything. He has the patience to endure whatever comes along.