Alexander Fernando Pacheco (born August 1958) is an American animal rights activist. He is the founder of 600 Million Dogs, co-founder and former chairman of People for the Ethical Treatment of Animals (PETA) and a member of the advisory board of the Sea Shepherd Conservation Society.

Pacheco first crewed with Captain Paul Watson in 1979 on the ship Sea Shepherd across the Atlantic Ocean, during a campaign of opposition to the Sierra, a Portuguese pirate whaling ship. Both The Sea Shepherd and the Sierra were sunk after being seized by the Portuguese authorities.

Pacheco came to wider public attention in 1981 for his role, along with Ingrid Newkirk, in what became known as the Silver Spring monkeys case, a campaign to release 17 crab-eating macaques who were undergoing experiments in the Institute for Behavioral Research in Silver Spring, Maryland. Filmmaker Oliver Stone writes that the political campaign to save the monkeys gave birth to the animal rights movement in the United States.

Pacheco was born in Joliet, Illinois, but moved to Mexico with his family when he was very young, where he and his two siblings were raised near the ocean by his Mexican father, a physician, and his mother, an American nurse. Kathy Snow Guillermo writes in Monkey Business (1993) that Pacheco's early life was filled with animals; bats lived in the rubber trees in his front yard, snakes slept behind nearby rocks, and fishermen regularly dragged dolphins out of the water onto the beach. Instead of animals being killed for food in slaughterhouses, pigs, oxen, chickens, and turkeys were frequently killed in front of him.

The family left Mexico when Pacheco was in junior high, and moved between Ohio, Indiana, and Illinois. His interest in animals continued. He bought turtles and birds from pet stores, and a baby crab-eating macaque, whom he called Chi Chi and who would perch on his shoulder as he walked around the house.

He attended a Catholic university in Ohio, intending to enter the priesthood, but while in Canada in his first year at the university, he visited a friend who worked at a meat-packing plant. Guillermo writes that he was shocked by the sight of two men throwing a newborn calf, cut from the uterus of its slaughtered mother, into a dumpster. Later in the week, a friend gave him a copy of Peter Singer's Animal Liberation, and he returned to Ohio as a vegetarian. His heart was no longer in becoming a priest, and he decided to attend Ohio State University in Columbus, Ohio and devote himself to helping what he called "other-than-human beings".

At university, Pacheco organized campaigns against the use of leghold traps and castrating pigs and cattle without anesthetic. Guillermo writes that, as Ohio is an agricultural state, his activism met with stiff opposition and the occasional anonymous telephone call threatening to blow his head off.

In 1979, he attended a talk in Columbus, Ohio, by Cleveland Amory of the Saturday Review, who was also the founder of the Fund for Animals, which funded the anti-whaling vessel the Sea Shepherd. He sought Amory out after the talk and volunteered. Pacheco first crewed with Paul Watson on the ship for the summer in 1979 (and again in 2003), in the bridge, the engine room and as a deckhand, during the Sea Shepherd's first whale protection campaign, known as The Sierra Campaign, across the Atlantic, which ended with both the Sea Shepherd and the Sierra being sunk, in Portugal in 1980.