Masoe, 26, is currently in his second season with St Helens in the Super League after helping them to a premiership in 2014.
With key forward Trent Merrin heading to Penrith at the end of the year, Mose will bring some much-needed size and depth to the forward pack in 2015.
"Mose will be a welcome addition to the club as he is able to provide both the additional size and go-forward which is vital in all strong forward-packs," said Dragons director of recruitment and retention Peter Mulholland.
"He is presently playing his trade in the traditionally-physical Super League competition and is a key player for the strong St Helens club."
Masoe made his NRL debut for the Roosters in 2010 and played one season with the Panthers before heading to England last year.
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