Yes, TREADMILL is a Scrabble word!

Letter values in Scrabble US: T 1, R 1, E 1, A 1, D 2, M 3, I 1, L 1, L 1.

Is it a valid word?

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

an exercise device consisting of an endless belt on which a person can walk or jog without changing place
a mill that is powered by men or animals walking on a circular belt or climbing steps
a job involving drudgery and confinement

Also: tread-wheel, salt mine.

Not playable in Scrabble US: TREADWHEEL.