Yes, PENTAGON is a Scrabble word!

Letter values in Scrabble US: P 3, E 1, N 1, T 1, A 1, G 2, O 1, N 1.

Is it a valid word?

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

a government building with five sides that serves as the headquarters of the United States Department of Defense
the United States military establishment
a five-sided polygon