Yes, HUMANISM is a Scrabble word!

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

Is it a valid word?

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

the doctrine that people's duty is to promote human welfare
the doctrine emphasizing a person's capacity for self-realization through reason; rejects religion and the supernatural
the cultural movement of the Renaissance; based on classical studies

Also: secular humanism.