Yes, TIS is a Scrabble word!

Letter values in Scrabble US: T 1, I 1, S 1.

Is it a valid word?

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

a light strong grey lustrous corrosion-resistant metallic element used in strong lightweight alloys (as for airplane parts); the main sources are rutile and ilmenite
shrub with terminal tufts of elongated leaves used locally for thatching and clothing; thick sweet roots are used as food; tropical southeastern Asia, Australia and Hawaii
the syllable naming the seventh (subtonic) note of any musical scale in solmization

Also: atomic number 22, cordyline terminalis.

Not playable in Scrabble US: TE.