Yes, RECIDIVIST is a Scrabble word!

Letter values in Scrabble US: R 1, E 1, C 3, I 1, D 2, I 1, V 4, I 1, S 1, T 1.

Is it a valid word?

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

someone who is repeatedly arrested for criminal behavior (especially for the same criminal behavior)
someone who lapses into previous undesirable patterns of behavior

Also: habitual criminal.

Not playable in Scrabble US: REVERSIONIST.