Plymouth is a city and the county seat of Marshall County, Indiana, United States. The population is 10,214 in the 2020 census. Plymouth was the site of the first retail outlet of defunct U.S. retailer Montgomery Ward in 1926.

This area was part of the territory of the Potawatomi Native Americans, one of the historical tribes encountered by American settlers. In the nineteenth century, the United States government made numerous treaties to buy and extinguish Native American claims to land in the former Northwest Territory and the Southeast.

Marshall County was formed in 1836, during the early years of settlement and before the forced removal of the Potawatomi people in 1838. It was named for U.S. Chief Justice John Marshall, who died in 1835. Marshall County is notable as the starting point in 1838 of the Potawatomi Trail of Death, which was the forced removal by United States forces of Chief Menominee and 859 Potawatomi Indians from Indiana to Indian Territory, at the site of present-day Osawatomie, Kansas, a distance of 660 miles (1,060 km). The first settlers arrived in what is now Marshall County in 1835. They arrived as a result of the end of the Black Hawk War as well as the completion of the Erie Canal. They consisted primarily of settlers from New England, "Yankees" descended from the English Puritans who settled New England in the colonial era. They were mainly members of the Congregational church, although due to the Second Great Awakening many of them had converted to Methodism and some had become Baptists before coming to what is now Marshall County. As a result of this heritage, some place names in Marshall County are named after places in New England, such as Plymouth, which is named after Plymouth, Massachusetts, the site where the Mayflower landed in 1620.

Plymouth, located near the center of Marshall County, was established on both banks of the Yellow River, a stream flowing from northeast to southwest across the county. In 1836, a dam and sawmill were constructed on the river. Many of the original settlers were immigrants from Germany, and mostly practiced farming. Upon their arrival, the land was covered with dense forests and marshlands, which required clearing and drainage before it could be cultivated. Early crops grown in Marshall County included corn, wheat, oats, rye, and beans.

The Indian Removal Act of 1830 authorized the forcible removal of Native American tribes from this area. In 1832, the United States government acquired the land through the Treaty of Tippecanoe.

Plymouth became the county seat of Marshall County in 1836 and established its first post office in 1837. The town was officially incorporated in 1851, and by 1900, its population had grown to 3,656, making it the largest town in the county.

Although many Potawatomi had relocated to Indian Territory in present-day Kansas, Chief Menominee and his band of the Yellow River refused to go. Militia under the authority of United States Army officers rounded them up and in September 1838, the band of 859 headed west, in what became known as the Potawatomi Trail of Death. They traveled more than 660 miles to Osawatomie, Kansas, with many of the Potawatomi walking. More than 40 of the tribal members died on the march.

During the Civil War, Union Army soldiers arrested Daniel E. VanValkenburgh, editor of the Plymouth Weekly Democrat, who criticized what he viewed as Lincoln's abuses of power. VanValkenburgh also criticized Department of Ohio commander Ambrose Burnside's lieutenant, General Milo S. Hascall. Hascall was in charge of the District of Indiana. VanValkenburgh called Hascall a donkey in the pages of the Democrat. Hascall promptly dispatched soldiers to arrest VanValkenburgh and brought him before Burnside to answer charges of violating Burnside's General Order No. 38. Burnside let VanValkenburgh off with a warning. The Democrat became the first of eleven Democratic newspapers suppressed or threatened with suppression in May 1863.