Yes, MAJUSCULES is a Scrabble word!

Letter values in Scrabble US: M 3, A 1, J 8, U 1, S 1, C 3, U 1, L 1, E 1, S 1.

Is it a valid word?

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

one of the large alphabetic characters used as the first letter in writing or printing proper names and sometimes for emphasis; capitals were kept in the upper half of the type case and so became known as upper-case letters"

Also: capital letter, upper-case letter.