Stadler Rail AG is a Swiss manufacturer of railway rolling stock based in Bussnang, Switzerland. Established with a focus on regional multiple unit trains and trams, the company has since diversified into the production of rapid transit, high-speed rail, intercity, and sleeper train vehicles. Stadler is also among the few remaining manufacturers of rack railway rolling stock. As of 2023, the company employed approximately 13,900 people worldwide.

Stadler Rail operates through 50 subsidiaries across 23 countries, including Algeria, Germany, Italy, the Netherlands, Austria, Poland, Spain, Czech Republic, Hungary, Belarus, and the United States. The company has also established joint ventures with INKA in Indonesia and Medha Servo Drives in India.

Stadler Rail traces its origins back to an engineering office established by Ernst Stadler (1908–1981) in 1942. Three years later, the company began to manufacture its first locomotives, building both battery-electric and diesel types. Throughout the majority of Stadler Rail's existence, it operated as a relatively small family-owned business entirely based in Switzerland that traditionally focused on manufacturing highly customised rail vehicles for its clients. The customer base were typically within relatively niche markets, such as narrow gauge and mountain railway operators, rather than those operating conventional mainline railways. After Ernst Stadler died in 1981, his second wife, Irma (c. 1923–2020), took over as CEO. Stadler Rail remained a relatively small rolling stock manufacturer even through to the 1990s; by the mid-1990s, Stadler reportedly had only 100 employees.

Around 1984, Stadler Rail decided to embark on the manufacture of passenger rolling stock for the first time. In 1987, Peter Spuhler, an in-law relative of the Stadler family through his marriage to one of Ernst Stadler's granddaughters, joined the company and subsequently took over as CEO from Irma Stadler in 1989. Spuhler then decided to expand the business via the launch of new products, as well as the acquisition of two other Swiss factories that built specialist rail vehicles for rack-and-pinion and narrow gauge railways. Stadler Rail experienced a considerable uptick in business during the latter years of 1990s. Its customer base continued to expand year after year over the following two decades to become one of the fastest growing and most innovative train manufacturing companies operating in Europe. Stadler Rail has become a serious competitor in several categories to traditional major rolling stock companies, such as Alstom and Siemens, and has successfully secured several major orders from the incumbent train operating companies of several nations.

In 1999, Stadler Rail took a 67% shareholding in a joint venture with Adtranz to manufacture the Regio-Shuttle RS1. However, following Adtranz's acquisition by Bombardier in 2001, European Union regulators insisted on the divestiture of the regional and tram product lines. As a consequence, Stadler Rail took 100% ownership of the Pankow factory in Berlin, becoming its first manufacturing base in Germany, that same year. Production of the RS1 has continued, becoming the market leading tram in the nation.

Perhaps the company's most successful product has been the FLIRT (Fast Light Innovative Regional Train) family, the range includes highly diverse configurations to suit different needs, from smaller regional units to luxurious intercity trainsets, as well as broad gauge versions for Finland and former Soviet Union nations. During 2004, Stadler Rail delivered the first trainset to the Swiss Federal Railways. By 2019, in excess of 1,400 FLIRTs have been ordered by operators in 16 countries spread across Europe, the United States, Algeria and Azerbaijan.

To facilitate an expanded order book and wider customer base, the company has rapidly expanded its production capabilities. To serve the Central and Eastern European market alone, during 2005, a new assembly plant was built in Hungary, while another was completed in Poland in the following year; six years later, a third manufacturing site was established in Belarus. By late 2019, the firm reportedly employed in excess of 7,000 employees at various locations spread across 20 countries. Each year, hundreds of rail vehicles, including trams, locomotives and coaches, are completed by the firm. In addition to its manufacturing efforts, considerable business is derived from contracted maintenance and refurbishment programmes, which Stadler Rail provides to operators throughout Europe, the United States, the Middle East and the North African regions.

The company has also grown via numerous acquisitions, including the Swiss company Winpro AG based in Winterthur in 2005, Voith Rail Services of the Netherlands in 2013, and Vossloh Rail Vehicles España S.A. of Valencia during 2015. They have been integrated into the wider Stadler Rail organisation, broadening the range of products and services on offer.