Yes, NATIONALISMS is a Scrabble word!

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

Is it a valid word?

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

love of country and willingness to sacrifice for it
the doctrine that your national culture and interests are superior to any other
the aspiration for national independence felt by people under foreign domination
the doctrine that nations should act independently (rather than collectively) to attain their goals