Ugg boots are a unisex style of sheepskin boot or slipper originating in Australia. The boots are typically made of twin-faced sheepskin with fleece on the inside, a tanned outer surface and a synthetic sole. The term "ugg boots" originated in Australia, initially for utilitarian footwear worn for warmth, and which were often worn by surfers during the 1960s. In the 1970s, the boots were introduced to the surf culture of the United Kingdom and the United States. Sheepskin boots became a fashion trend in the U.S. in the late 1990s and a worldwide trend in the mid-2000s. In Australia, they are worn predominantly as slippers and often associated with daggy fashion sense and bogan culture.

Prior legal disputes between some manufacturers of sheepskin boots arose as to distinguish whether "ugg" is a protected trademark, or a generic term and thus ineligible for trademark protection. There are more than 70 registered trademarks that include the term "ugg" in various logos and designs in Australia and New Zealand, as the term is considered a generic reference to a type of shoe. Outside Australia and New Zealand, UGG is a brand manufactured by the California-based Deckers Outdoor Corporation, with most of its manufacturing based in China and with registered trademarks in over 130 countries worldwide including the US, UK, Canada, all European Union members, and China. Despite the difference in capitalisation and pronunciation (e.g., yew-gee-gee in China), Deckers Corporation actively asserts its "UGG" trade mark against Australian traders who sell their "ugg boots" overseas.

Noteworthy manufacturers in Australia and New Zealand are EMU Australia, Uggs-N-Rugs and Ugg Australia. While Deckers is often referred to as the leading manufacturer of the footwear style outside Australia, Deckers refers to its boots as the "UGG-brand" boot, and associates it with a "California lifestyle". Following an investigation by the Australian Competition & Consumer Commission into allegedly misleading and deceptive use of "Australia" in its branding, Deckers has rebranded from "UGG Australia" to "UGG".

There are different claims to the origins of the ugg boot style. Artisanal sheepskin boots were known in rural Australia during the 1920s, and were reportedly worn by shearers as they found them resistant to wool yolk (lanolin), which would rot their ordinary boots. However, when commercial manufacturing began remains unclear. The boots were reportedly being manufactured in 1933 by Blue Mountains Ugg Boots of New South Wales. Frank Mortel of Mortels Sheepskin Factory has stated that he began manufacturing the boots in the late 1950s. Surfer Shane Stedman of Australia has stated in interviews that he invented the ugg boot. Perth sheepskin boot manufacturers Bruce and Bronwyn McDougall of Uggs-N-Rugs have manufactured the boots since the late 1970s.

The origin of the term "ugg" is also unclear. Stedman registered the trademark "UGH-BOOTS" in Australia in 1971, and in 1982 registered the "UGH" trademark. Frank Mortel claims that he named his company's sheepskin boots "ugg boots" in 1958 after his wife commented that the first pair he made were "ugly." Some accounts have suggested that the term grew out of earlier variations, such as the "fug boots" worn by Royal Air Force pilots during World War I.

The 1970s saw the emergence of advertising using the UGG and UGH terms both in trade names and as a generic term in Australia. The Macquarie Dictionary of the Australian language first included a definition for "ugg boot" as a generic term for sheepskin boots in its 1981 edition. (After Stedman complained to the editors of Macquarie, subsequent editions indicated that "UGH" was a trade mark.)

In the 1970s, ugg boots became popular among competitive surfers. Sheepskin footwear accounts for around 10 percent of footwear production in Australia.

Surfing helped popularise the boots outside Australia and New Zealand. Advertisements for Australian sheepskin boots first appeared in Californian surf magazines in 1970. By the mid-1970s, several surf shops in Santa Cruz, California and the San Fernando Valley were selling a limited number of boots purchased by the shops' owners while visiting surfing events in Australia. In 1978, a Western Australian manufacturer of sheepskin boots, Country Leather, advertised outside Australia for distributors to sell its sheepskin boots, which were made from sheepskin sourced from Jackson's Tannery in Geelong, Victoria. Seeing the popularity of the boots among American surfers, Australian surfer Brian Smith, then living in Santa Monica, California, and colleague Doug Jensen bought boots from Country Leather but were unhappy with the brand and decided to register UGG as their own trademark. Family friends invested $20,000 into the new venture and the group set up Ugg Imports. Due to other business commitments, in 1979 Jensen handed over his share of the company to Smith. In 1987 Smith registered Ugg Holdings Inc. and in 1985 registered a U.S. trademark on a ram's head logo with the words "Original UGG Boot UGG Australia." In 1995, Ugg Holdings purchased Stedman's various trademarks. As for the ugg name, Smith stated: "We always called them uggs, long before it was trademarked."