Seguin (/sɪˈɡiːn/ sih-GHEEN) is a city in and the county seat of Guadalupe County, Texas, United States. Its population was 29,433 at the 2020 census, and according to 2023 census estimates, the city had a population of 36,013.

Seguin's economy is primarily supported by a regional hospital, as well as the Schertz-Seguin Local Government Corporation water utility that supplies the surrounding greater San Antonio areas from nearby aquifers as far as Gonzales County. Several dams in the surrounding area are governed by the main offices of the Guadalupe-Blanco River Authority, headquartered in downtown Seguin.

Seguin, named in honor of Juan Seguín, a Tejano Texian freedom fighter and early supporter of the Republic of Texas, is one of the oldest towns in Texas, founded just 16 months after the Texas Revolution began. The frontier settlement was a cradle of the Texas Rangers and home to celebrated Captain Jack Hays, perhaps the most famous Ranger of all. At that time, the Seguin area was a part of Gonzales County, the remaining portion known as present-day Belmont. The Rangers had found this was a good halfway stop between their patrol points. It had been maintained as a base camp by the Rangers since the early founding of the Dewitt Colony.

Seguin was the home of Dr. John E. Park, who experimented in construction using concrete made from local materials. The nearly 100 structures—the courthouse, schools, churches, homes, cisterns, walls, etc.—made up the largest concentration of early 19th-century concrete buildings in the United States. About 20 of them remain standing.

The use of concrete largely ended when the railroad arrived in 1876, bringing cheap lumber and the equipment needed for brickmaking. The town had five brickworks, and the wooden buildings of downtown were completely replaced with brick by the beginning of World War I.

For almost 100 years, the town was dependent on the rich surrounding farmland and ranches. Then, the Texas oil boom came just as the Great Depression was taking down other towns and cities. The town commemorated its centennial by opening Max Starcke Park, with a golf course, a pavilion, picnic tables, and BBQ pits along a scenic river drive, and a curving dam that created a waterfall.

To preserve some of the historic character of the town, Seguin became one of the state's first Main Street cities, and the downtown district was placed on the National Register of Historic Places. Fine homes by leading architects J. Reily Gordon, Solon McAdoo, Leo M.J. Dielman, Atlee B. Ayers, and Marvin Eickenroht dating from the late 19th century and the first half of the 20th century can be found on many streets. However, the city does not have any officially designated historic residential districts.

The postwar era had industrial development, including a small mill that turned scrap metal into construction products.