Yes, SOUTHERN is a Scrabble word!

Letter values in Scrabble US: S 1, O 1, U 1, T 1, H 4, E 1, R 1, N 1.

Is it a valid word?

SOUTHERN is a valid word in the official Scrabble dictionary (US), the UK Scrabble Dictionary and Words with Friends (ENABLE).

in or characteristic of a region of the United States south of (approximately) the Mason-Dixon line
situated in or oriented toward the south
situated in or coming from regions of the south
from the south; used especially of wind