Yes, YET is a Scrabble word!

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

Is it a valid word?

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

up to the present time
used in negative statement to describe a situation that has existed up to this point or up to the present time
to a greater degree or extent; used with comparisons
within an indefinite time or at an unspecified future time
used after a superlative
despite anything to the contrary (usually following a concession)

Also: so far, thus far, up to now, as yet, til now, until now, in time, all the same, even so.