Yes, KEY is a Scrabble word!

Letter values in Scrabble US: K 5, E 1, Y 4.

Is it a valid word?

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

metal device shaped in such a way that when it is inserted into the appropriate lock the lock's mechanism can be rotated
something crucial for explaining
pitch of the voice
any of 24 major or minor diatonic scales that provide the tonal framework for a piece of music
a kilogram of a narcotic drug
a winged often one-seed indehiscent fruit as of the ash or elm or maple
United States lawyer and poet who wrote a poem after witnessing the British attack on Baltimore during the War of 1812; the poem was later set to music and entitled `The Star-Spangled Banner' (1779-1843)
a coral reef off the southern coast of Florida
(basketball) a space (including the foul line) in front of the basket at each end of a basketball court; usually painted a different color from the rest of the court
a list of answers to a test
a list of words or phrases that explain symbols or abbreviations
a generic term for any device whose possession entitles the holder to a means of access
mechanical device used to wind another device that is driven by a spring (as a clock)
the central building block at the top of an arch or vault
a lever (as in a keyboard) that actuates a mechanism when depressed

Also: key fruit, francis scott key, florida key, key out.