Yes, LIEUTENANT is a Scrabble word!

Letter values in Scrabble US: L 1, I 1, E 1, U 1, T 1, E 1, N 1, A 1, N 1, T 1.

Is it a valid word?

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

a commissioned military officer
an officer in a police force
an assistant with power to act when his superior is absent
an officer holding a commissioned rank in the United States Navy or the United States Coast Guard; below lieutenant commander and above lieutenant junior grade

Also: police lieutenant.