Yes, MESSIAHS is a Scrabble word!

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

Is it a valid word?

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

any expected deliverer
Jesus Christ; considered by Christians to be the promised deliverer
the awaited king of the Jews; the promised and expected deliverer of the Jewish people
an oratorio composed by Handel in 1742

Not playable in Scrabble US: CHRIST.