Yes, HUMPBACK is a Scrabble word!

Letter values in Scrabble US: H 4, U 1, M 3, P 3, B 3, A 1, C 3, K 5.

Is it a valid word?

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

an abnormal backward curve to the vertebral column
a person whose back is hunched because of abnormal curvature of the upper spine
large whalebone whale with long flippers noted for arching or humping its back as it dives

Also: humpback whale, megaptera novaeangliae.