Yes, PIRATES is a Scrabble word!

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

Is it a valid word?

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

someone who uses another person's words or ideas as if they were his own
someone who robs at sea or plunders the land from the sea without having a commission from any sovereign nation
a ship that is manned by pirates

Also: literary pirate, sea robber, sea rover, pirate ship.

Not playable in Scrabble US: PLAGIARISER.