Yes, SIGN is a Scrabble word!

Letter values in Scrabble US: S 1, I 1, G 2, N 1.

Is it a valid word?

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

a perceptible indication of something not immediately apparent (as a visible clue that something has happened)
a public display of a message
any nonverbal action or gesture that encodes a message
structure displaying a board on which advertisements can be posted
(astrology) one of 12 equal areas into which the zodiac is divided
(medicine) any objective evidence of the presence of a disorder or disease
having an indicated pole (as the distinction between positive and negative electric charges)
an event that is experienced as indicating important things to come
a gesture that is part of a sign language
a fundamental linguistic unit linking a signifier to that which is signified
a character indicating a relation between quantities

mark with one's signature; write one's name (on)
approve and express assent, responsibility, or obligation
be engaged by a written agreement
engage by written agreement
communicate silently and non-verbally by signals or signs
place signs, as along a road
communicate in sign language
make the sign of the cross over someone in order to call on God for protection; consecrate

Also: sign of the zodiac, star sign, planetary house, sign on, sign up, sign-language.

Not playable in Scrabble US: PREINDICATION.