Yes, HUMANISTS is a Scrabble word!

Letter values in Scrabble US: H 4, U 1, M 3, A 1, N 1, I 1, S 1, T 1, S 1.

Is it a valid word?

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

a classical scholar or student of the liberal arts
an advocate of the principles of humanism; someone concerned with the interests and welfare of humans