Yes, MILITIA is a Scrabble word!

Letter values in Scrabble US: M 3, I 1, L 1, I 1, T 1, I 1, A 1.

Is it a valid word?

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

civilians trained as soldiers but not part of the regular army
the entire body of physically fit civilians eligible by law for military service