Yes, WORK is a Scrabble word!

Letter values in Scrabble US: W 4, O 1, R 1, K 5.

Is it a valid word?

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

activity directed toward making or doing something
a product produced or accomplished through the effort or activity or agency of a person or thing
the occupation for which you are paid
applying the mind to learning and understanding a subject (especially by reading)
(physics) a manifestation of energy; the transfer of energy from one physical system to another expressed as the product of a force and the distance through which it moves a body in the direction of that force
a place where work is done
the total output of a writer or artist (or a substantial part of it)

exert oneself by doing mental or physical work for a purpose or out of necessity
be employed
have an effect or outcome; often the one desired or expected
perform as expected when applied
shape, form, or improve a material
give a workout to
proceed along a path
operate in a certain place, area, or specialty
proceed towards a goal or along a path or through an activity
move in an agitated manner
cause to happen or to occur as a consequence
cause to work
prepare for crops
behave in a certain way when handled
have and exert influence or effect
operate in or through
cause to operate or function
provoke or excite
gratify and charm, usually in order to influence
make something, usually for a specific function
move into or onto
make uniform
use or manipulate to one's advantage
find the solution to (a problem or question) or understand the meaning of
cause to undergo fermentation
go sour or spoil
arrive at a certain condition through repeated motion

Also: piece of work, body of work, do work, work on, work out, make for, put to work, act upon, figure out, puzzle out.