Weldon Spring is a city in St. Charles County, Missouri, United States. The population was 5,443 at the 2010 census.
Weldon Spring was platted in 1864. The community took its name from a spring of the same name near the original town site. A post office called Weldon Spring was established in 1875, and remained in operation until 1957.
In 1941, the United States Army purchased 17,000 acres of land in the surrounding area. The Weldon Spring Ordnance Works built the largest explosives factory in the United States on the site. It was built to produce TNT and DNT for Allied forces during World War II. It closed on August 15, 1945, days after the end of the war. Most of the land was then sold.
In 1955, the United States Atomic Energy Commission built a uranium ore processing plant on the 2,000 remaining acres.