Yes, PRIEST is a Scrabble word!

Letter values in Scrabble US: P 3, R 1, I 1, E 1, S 1, T 1.

Is it a valid word?

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

a clergyman in Christian churches who has the authority to perform or administer various religious rites; one of the Holy Orders
a person who performs religious duties and ceremonies in a non-Christian religion

Also: non-christian priest.