Yes, NATURALISM is a Scrabble word!

Letter values in Scrabble US: N 1, A 1, T 1, U 1, R 1, A 1, L 1, I 1, S 1, M 3.

Is it a valid word?

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

(philosophy) the doctrine that the world can be understood in scientific terms without recourse to spiritual or supernatural explanations
an artistic movement in 19th century France; artists and writers strove for detailed realistic and factual description