Yes, PANNIER is a Scrabble word!

Letter values in Scrabble US: P 3, A 1, N 1, N 1, I 1, E 1, R 1.

Is it a valid word?

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

either of a pair of bags or boxes hung over the rear wheel of a vehicle (as a bicycle)
a large basket (usually one of a pair) carried by a beast of burden or on by a person
set of small hoops used to add fullness over the hips