Arnold Line is an unincorporated area and census-designated place in Lamar County, Mississippi, United States. As of the 2020 census it had a population of 2,333. It is part of the Hattiesburg Metropolitan Statistical Area. It is in the northeast part of Lamar County and is bordered to the north, east, and south by the city of Hattiesburg. The center of Hattiesburg is 5 miles to the east in Forrest County. West Hattiesburg is 1 mile to the south of Arnold Line.