The American Locomotive Company, often shortened to ALCO or Alco, was a builder of railroad locomotives in the United States.

The company was created in 1901 from the merger of several smaller locomotive manufacturers:

The new company was headquartered in Schenectady and eventually closed all the other locomotive manufacturing plants, except for the main plants in Schenectady, New York, and Montreal, Quebec.

In 1904, control of the Locomotive and Machine Company of Montreal, Quebec, was acquired; this company was eventually renamed the Montreal Locomotive Works (MLW), and after over 50 years of closely working together, was to ultimately continue locomotive manufacturing operations after the U.S. parent company had ended production in 1969. The next year, 1905, Alco purchased Rogers Locomotive Works of Paterson, New Jersey, the second largest locomotive manufacturer in the U.S. behind Baldwin Locomotive Works.

Alco was the second-largest steam locomotive builder in the United States (after Baldwin), producing over 75,000 locomotives. Among these were a large number of well-known locomotives. Railroads that favored Alco products included the Delaware and Hudson Railway, the New York, New Haven and Hartford Railroad, the New York Central Railroad, the Union Pacific Railroad and the Southern Pacific. Alco was known for its steam locomotives of which the 4-6-4 Hudson and the 4-8-4 Niagara built for the New York Central and the 4-6-6-4 (Challenger) built for the Union Pacific were fine examples. Alco built many of the biggest locomotives ever constructed, including Union Pacific's Big Boy (4-8-8-4).