Yes, SEPARATISM is a Scrabble word!

Letter values in Scrabble US: S 1, E 1, P 3, A 1, R 1, A 1, T 1, I 1, S 1, M 3.

Is it a valid word?

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

a social system that provides separate facilities for minority groups
a disposition toward schism and secession from a larger group; the principles and practices of separatists
advocacy of a policy of strict separation of church and state

Not playable in Scrabble US: SEPARATIONISM.