Marlin is a city in Falls County, Texas, United States. The population was 6,628 at the 2000 census, but decreased to 5,967 by 2010. It is also the (third) county seat of Falls County, and has been so since 1851. Marlin has been given the nickname the Hot Mineral Water City of Texas when mineral waters were found in 1892. The Texas Department of Criminal Justice (TDCJ) operates the Marlin Unit, a transfer facility for men, in the City of Marlin. The unit opened in June 1992 and was transferred to the Texas Youth Commission (TYC) in May 1995. When it was a part of TYC, the facility, named the Marlin Orientation and Assessment Unit, served as the place of orientation for children of both sexes being committed into TYC from the facility's opening in 1995 to its transfer out of TYC in 2007.