Yes, PLAGIARISM is a Scrabble word!

Letter values in Scrabble US: P 3, L 1, A 1, G 2, I 1, A 1, R 1, I 1, S 1, M 3.

Is it a valid word?

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

a piece of writing that has been copied from someone else and is presented as being your own work
the act of plagiarizing; taking someone's words or ideas as if they were your own

Not playable in Scrabble US: PLAGIARIZATION, PLAGIARISATION.