Jefferson County is a county located in the Southeast Texas subregion of the U.S. state of Texas. As of the 2010 census, the population was 252,273. The county is part of the Beaumont–Port Arthur, TX Metropolitan Statistical Area and has the highest population of the four county MSA. The county seat of Jefferson County is Beaumont. The county was established in 1836 and was organized in 1837. It is named for U.S.