MS St. Louis was a diesel-powered passenger ship properly referred to with the prefix MS or MV, built by the Bremer Vulkan shipyards in Bremen for HAPAG, better known in English as the Hamburg America Line. The ship was named after the city of St. Louis, Missouri. Her sister ship, MS Milwaukee, was also a diesel powered motor vessel owned by the Hamburg America Line. St. Louis regularly sailed the trans-Atlantic route from Hamburg to Halifax, Nova Scotia, and New York City, and made cruises to the Canary Islands, Madeira, Spain; and Morocco. St. Louis was built for both transatlantic liner service and for leisure cruises.

The Hamburg-Amerikanische Packetfahrt-Actien-Gesellschaft (HAPAG), known in English as the Hamburg America Line, was a transatlantic shipping enterprise established in Hamburg, in 1847. Among those involved in its development were prominent citizens such as Albert Ballin, Adolph Godeffroy, Ferdinand Laeisz, Carl Woermann, August Bolten, and others, and its main financial backers were Berenberg Bank and H. J. Merck & Co. It soon developed into the largest German, and at times the world's largest, shipping company, serving the market created by German immigration to the United States and later, immigration from Eastern Europe. On 1 September 1970, after 123 years of independent existence, HAPAG merged with the Bremen-based North German Lloyd to form Hapag-Lloyd AG.