Yes, ADIEUS is a Scrabble word!

Letter values in Scrabble US: A 1, D 2, I 1, E 1, U 1, S 1.

Is it a valid word?

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

Also: auf wiedersehen, au revoir, bye-bye, good-by, good-bye, good day, so long.

Not playable in Scrabble US: ARRIVEDERCI.