Defiance is an unincorporated community in Saint Charles County, Missouri, located on state highway 94.
The Katy Trail State Park runs through Defiance near the Missouri River. This was the final home of frontiersman Daniel Boone, who settled in the Femme Osage Valley in 1799 after receiving a Spanish land grant. The hamlet was not named during Boone's life. Its population in 2000 was 3,154.
It was named in the late 1800s when the Katy Railroad arrived. Initially the town was to be called Parsons for the landowner. Because there was already a Parsons, Kansas on the Katy line, settlers considered other names, including Missouriton and Bluff City, before deciding on Defiance because of the hamlet's defiance of rival Matson, Missouri to get a station on the line.