Yes, SYMPTOM is a Scrabble word!

Letter values in Scrabble US: S 1, Y 4, M 3, P 3, T 1, O 1, M 3.

Is it a valid word?

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

(medicine) any sensation or change in bodily function that is experienced by a patient and is associated with a particular disease
anything that accompanies X and is regarded as an indication of X's existence