Yes, RATIONALISMS is a Scrabble word!

Letter values in Scrabble US: R 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?

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

(philosophy) the doctrine that knowledge is acquired by reason without resort to experience
the theological doctrine that human reason rather than divine revelation establishes religious truth
the doctrine that reason is the right basis for regulating conduct