Jefferson County is a county located in the U.S. state of Texas. As of the 2010 census, the population was 252,273. The county seat of Jefferson County is Beaumont. The county was established in 1835 as a municipality of Mexico and was organized as a county in 1837. It is named for U.S. president Thomas Jefferson.Jefferson County is part of the Beaumont–Port Arthur, TX Metropolitan Statistical Area and has the highest population of the four county MSA. It is located in Southeast Texas.