Yes, ARGONAUT is a Scrabble word!

Letter values in Scrabble US: A 1, R 1, G 2, O 1, N 1, A 1, U 1, T 1.

Is it a valid word?

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

someone engaged in a dangerous but potentially rewarding adventure
(Greek mythology) one of the heroes who sailed with Jason in search of the Golden Fleece
cephalopod mollusk of warm seas whose females have delicate papery spiral shells

Also: paper nautilus, argonauta argo.