Arnold Line is an unincorporated area and census-designated place (CDP) 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 (8 km) to the east in Forrest County. West Hattiesburg is 1 mile (1.6 km) to the south of Arnold Line.

Historical population

Census	Pop.	Note	%±
2010	1,719		—
2020	2,333		35.7%
U.S. Decennial Census 2010 2020

Arnold Line CDP, Mississippi – Racial and ethnic composition
Note: the US Census treats Hispanic/Latino as an ethnic category. This table excludes Latinos from the racial categories and assigns them to a separate category. Hispanics/Latinos may be of any race.

Race / Ethnicity (NH = Non-Hispanic)	Pop 2010	Pop 2020	% 2010	% 2020
White alone (NH)	966	955	56.20%	40.93%
Black or African American alone (NH)	670	1,118	38.98%	47.92%
Native American or Alaska Native alone (NH)	1	2	0.06%	0.09%
Asian alone (NH)	32	53	1.86%	2.27%
Pacific Islander alone (NH)	2	0	0.12%	0.00%
Some Other Race alone (NH)	2	16	0.12%	0.69%
Mixed Race or Multi-Racial (NH)	12	78	0.70%	3.34%
Hispanic or Latino (any race)	34	111	1.98%	4.76%
Total	1,719	2,333	100.00%	100.00%

It is in the Lamar County School District.