Daviess County (/ˈdeɪvɪs/ DAY-viss) is a county located in the U.S. state of Missouri. As of the 2020 census, the population was 8,430. Its county seat is Gallatin. The county was organized December 29, 1836, from Ray County and named for Major Joseph Hamilton Daveiss, a soldier from Kentucky who was killed in 1811 at the Battle of Tippecanoe.

The county includes the town of Jamesport, which has the largest Amish community in Missouri.

Latter Day Saint movement founder Joseph Smith taught that Adam-ondi-Ahman, in the central part of the county, was where Adam and Eve moved after being banished from the Garden of Eden. According to LDS tradition, the site is to be a gathering spot before the Second Coming of Jesus Christ.

In 1838, two years after the county was organized, Smith's claims about the area drew an influx of Mormon settlers. Non-Mormon residents feared they were going to lose control of the county and attempted to prevent Mormons from voting in the Gallatin Election Day Battle—the first skirmish in the Mormon War. Later, in retaliation for violence to their families and destruction of their property, some Mormons burned and sacked Gallatin, Grindstone Fork, Millport and other smaller settlements. The plundered goods were deposited in the Bishop's storehouse at Diahman. Millport, which at the time was the largest city in the county and the center for trade, never recovered, and became a ghost town. Missouri Governor Lilburn Boggs issued an Extermination Order to drive the Mormons from the state after arresting Joseph Smith and other leaders of the church.

Daviess County played a major role in the history of the outlaw James-Younger Gang. The first confirmed bank robbery involving Jesse James occurred on December 7, 1869, at the Daviess County Savings Association in Gallatin. During the robbery, James killed cashier John W. Sheets, whom he believed was Samuel P. Cox, who had killed James's bushwhacker colleague Bloody Bill Anderson during the American Civil War. The gang is also believed responsible for the July 15, 1881, robbery of the Rock Island Line at Winston in which a conductor and passenger were killed.

After Jesse James was murdered in St. Joseph, Frank James surrendered in 1882 to face Daviess County charges in connection with the train robbery/murder and murder charges in the 1869 bank robbery. His trial lasted from August 20 until September 6, 1883. Interest was so intense that the trial was moved to the Gallatin Opera House to accommodate the crowds. James was found not guilty of involvement in both crimes. Charges were made that the jury was filled with Southern sympathizers who refused to convict one of their own.

The Daviess County Savings Association and the Gallatin Opera House have since been torn down, although the Winston Rock Island Line train station still stands and is operated by the local historical society as a museum.

Daviess County has one of only three Rotary Jails still in existence. Also known as the "Squirrel Cage Jail," it is now a museum and is on the National Register of Historic Places.