The young open side, is currently with the Ospreys regional team having previously played for Swansea RFC and will join the Dragons from June.
On signing the fresh new talent, director of rugby Lyn Jones said, "I have always been a big fan of Sam and his brother before him.
"He comes from a good rugby family and has the edge to become a quality international player. Playing regularly will help him achieve his ambitions.
"This is a significant signing for us and just shows our determination to become the best region in Wales."
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