Yes, AMPHITHEATER is a Scrabble word!

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

Is it a valid word?

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

a sloping gallery with seats for spectators (as in an operating room or theater)
an oval large stadium with tiers of seats; an arena in which contests and spectacles are held

Not playable in Scrabble US: AMPHITHEATRE.