Wham-O Inc. is an American toy company based in Carson, California, United States. It is known for creating and marketing many popular toys for nearly 70 years, including the Hula hoop, Frisbee, Slip 'N Slide, Super Ball, Trac-Ball, Silly String, Hacky Sack, Wham-O Bird Ornithopter, and Boogie Board, many of which have become genericized trademarks.

Richard Knerr (1925–2008) and Arthur "Spud" Melin (1924–2002), two University of Southern California graduates who were friends since their teens, were unhappy with their jobs and decided to start their own business. In 1948, they formed the WHAM-O Manufacturing Company in the Knerr family garage in South Pasadena. Their first product was the Wham-O Slingshot, made of ash wood, which Knerr and Melin promoted by holding demonstrations of their own slingshot skills. The name "Wham-O" was inspired by the sound of the slingshot's shot hitting the target. The powerful slingshot was adopted by clubs for competitive target shooting and small game hunting. When they outgrew the garage, Knerr and Melin rented a building on S. Marengo Ave in Alhambra, California. In 1955, they moved their manufacturing plant to neighboring San Gabriel, California, where they remained until 1987, when they sold the plant to Huy Fong Foods.

In 1958, Wham-O, still a fledgling company, took the idea of Australian bamboo "exercise hoops", manufactured them in Marlex, and called their new product the Hula Hoop. The name had been used since the 18th century, but until then was not registered as a trademark. It became the biggest toy fad in modern history. 25 million were sold in four months, and in two years sales reached more than 100 million. "Hula Hoop mania" continued through the end of 1959, and netted Wham-O $45 million (equivalent to $485 million in 2024).

Shortly thereafter, the company had another huge success with the Frisbee. In 1955, inventor Fred Morrison began marketing a plastic flying disc called the Pluto Platter. He sold the design to Wham-O on January 23, 1957. By June they had learned that students back east were calling them a "Frisbee." In early 1958, Wham-O added the name "Frisbee" to the top of the Pluto Platter – and once again a Wham-O toy became a common part of life through the 1960s.

In the early 1960s, Wham-O created the Super Ball, a high-bouncing ball made of a hard elastomer Polybutadiene alloy, dubbed Zectron, with a 0.92 coefficient of restitution when bounced on hard surfaces. Around 20 million Super Balls were sold that decade, and the NFL named the Super Bowl games after it.

The Frisbee and Hula Hoop created fads. With other products, Wham-O tried to capitalize on existing national trends. In the 1960s, they produced a US$119 do-it-yourself bomb shelter cover. In 1962, they sold a limbo dance kit to take advantage of that fad; and in 1975, when the movie Jaws was released, they sold plastic shark teeth.

Many products were not successful. During an African safari in the early 1960s, Melin discovered a species of fish that laid eggs in the mud during Africa's dry season. When the rains came, the eggs hatched and fish emerged overnight. This inspired Melin to create the Instant Fish product, an aquarium kit consisting of some of the fish eggs, and some mud in which to hatch them.

Knerr and Melin marketed their products directly to children, including demonstrating their toys at playgrounds. They extensively researched new product ideas, including traveling around the world. "If Spud and I had to say what we contributed," Knerr said, "it was fun. But I think this country gave us more than we gave it. It gave us the opportunity to do it."