Yes, REALIST is a Scrabble word!

Letter values in Scrabble US: R 1, E 1, A 1, L 1, I 1, S 1, T 1.

Is it a valid word?

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

a philosopher who believes that universals are real and exist independently of anyone thinking of them
a person who accepts the world as it literally is and deals with it accordingly
a painter who represents the world realistically and not in an idealized or romantic style