 Jefferson County is on the plains of the Texas Gulf Coast in the southeastern part of the state.It is bounded on the north by Pine Island Bayou, on the northeast by the Neches River, and on the east by Sabine Lake and the mouth of the Sabine River, a natural outlet called Sabine Pass.The southern part of the county is largely marshland, much of which is within Sea Rim State Park, reaching the storm-battered beach at the Gulf of Mexico.