Yes, ADJACENT is a Scrabble word!

Letter values in Scrabble US: A 1, D 2, J 8, A 1, C 3, E 1, N 1, T 1.

Is it a valid word?

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

nearest in space or position; immediately adjoining without intervening space
having a common boundary or edge; abutting; touching; Massachusetts and Conncecticut"
near or close to but not necessarily touching

Also: side by side.