The St. Louis, Iron Mountain and Southern Railway was a historic railroad that operated in Missouri, and Arkansas during the late 19th and early 20th centuries.

It ran from St. Louis, Missouri, to Taxarkana, Arkansas, as well as to southeast Missouri. The line was initially established to deliver iron ore from Iron Mountain Missouri to St. Louis. The company was frequently referred to as the St. Louis Iron Mountain & Southern (StLIM&S) or shortened to simply the Iron Mountain Railway.

The railroad was robbed twice, once by the James-Younger Gang on January 31, 1874, at Gad's Hill, Missouri, and once by the Dalton Gang on November 3, 1893, at Olyphant, Arkansas.

In 1883 the StLIM&S was acquired by Jay Gould, becoming part of a 9,547 miles (15,364 km) system. On May 12, 1917, it was formally merged into the Missouri Pacific Railroad, which in turn was merged into the Union Pacific Railroad in 1982.