Is brain a word in the Scrabble Dictionary

Yes, brain is playable

...and is worth 9 points.

Found in the following dictionaries

Enable (Words with friends)
SOWPODS (Collins Scrabble Words)
OTCWL2014 (NASPA Word List 2014)
TWL06 (NASPA List 2006)

"Brain" Definition

noun

A functionally similar portion of the invertebrate nervous system.
A highly intelligent person.
Intellectual ability; mind: a dull brain; a quick brain.
Intellectual power; intelligence. Often used in the plural: has brains and good looks. See Synonyms at mind.
The control center, as of a ship, aircraft, or spacecraft.

idiom

On the brain Obsessively in mind: The coach has winning on the brain.

verb-transitive

Slang To hit on the head.
Slang To smash in the skull of.

verb

To kill by smashing the skull.
To hit on the head.