Is theirs a word in the Scrabble Dictionary

Yes, theirs is playable

...and is worth 8 points.

Found in the following dictionaries

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

"Theirs" Definition

pronoun

Usage Problem His or hers: brought his own lunch and expected everybody else to bring theirs. See Usage Notes at he1, they.

pronoun, singular or plural in construction

That which belongs to them .
His or hers .