Gillig (formerly Gillig Brothers) is an American designer and manufacturer of buses. The company headquarters, along with its manufacturing operations, is located in Livermore, California (in the East Bay region of the San Francisco Bay Area). By volume, Gillig is the second-largest transit bus manufacturer in North America (behind New Flyer). As of 2013, Gillig had an approximate 31 percent market share of the combined United States and Canadian heavy-duty transit bus manufacturing industry, based on the number of equivalent unit deliveries.

While currently a manufacturer of transit buses, from the 1930s to the 1990s, Gillig was a manufacturer of school buses. Alongside the now-defunct Crown Coach, the company was one of the largest manufacturers of school buses on the West Coast of the United States. During the 1960s and the 1970s, Gillig was a West Coast distributor for other manufacturers of conventional-style buses, including Superior Coach Company and Thomas Built Buses. Gillig had been located in Hayward, California, for more than 80 years before moving to Livermore in 2017. The company was founded in San Francisco, by the Gillig brothers.

The oldest surviving bus manufacturer in North America, Gillig was founded in 1890 as Jacob Gillig, trained in carriage building and upholstering, opened his own carriage shop in San Francisco. In 1896, his son Leo Gillig entered the business as a shop foreman, becoming a full partner in the business in 1900. The shop was destroyed as part of the 1906 San Francisco earthquake, but the Gilligs rebuilt the shop on a separate property; Chester Gillig joined the business as a bookkeeper. In 1907, Jacob Gillig died at the age of 54.

Following the earthquake, the company reopened as the Leo Gillig Automobile Works, which manufactured custom-built vehicle bodies. In 1914, two major achievements would happen to the company. After building a three-story factory, Leo and Chester Gillig re-organized the company as Gillig Brothers, its name for the next half-century. One of the first bodies built inside the new factory was one for a motor bus, though production would not shift entirely to buses for another two decades.

During the 1910s, most cars in the United States were open touring cars; at the time, fully enclosed sedan bodies were expensive. To offer improvement over the minimal weather protection, Gillig developed an add-on hardtop, patenting its own version in 1919.

The increase of closed car production in the 1920s would render the "Gillig Top" largely obsolete by 1925. While other hardtop manufacturers went out of business, Gillig survived largely on its body production, which became its primary source of revenue. In the late 1920s, the company would briefly produce pleasure boats and produce a prototype of a heavy truck; the latter would never enter production.

Following the start of the Great Depression, Gillig Brothers began to look for a steady source of revenue to ensure its survival. Although the company had produced buses sporadically since 1914, in 1932, Gillig designed its first school bus body, a configuration it would produce for most of the next 60 years. In 1935, the company designed its first ambulance body; it also became the West Coast distributor of Superior Coach Company, a manufacturer of school buses and professional cars. In 1937, Gillig introduced its first flat-front (transit-style) school bus.

By 1938, demand for school buses had surpassed the capacity of the San Francisco facility, leading Gillig Brothers to move to Hayward, California, on the eastern side of San Francisco Bay.