"This is a significant signing for us and just shows our determination to become the best region in Wales," said Dragons director of rugby Lyn Jones.
"I have always been a big fan of Sam. 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."
Lewis has played in all four of the Ospreys' Heineken Cup fixtures so far this season, matching his involvement from last term in what was his first season of European competition.
The former Swansea RFC star has featured 18 times in all in his third year as a professional but faces huge competition for the No7 shirt from British & Irish tourist Justin Tipuric and has often been employed on the blindside flank as a result.
And while Lewis is highly rated by the Ospreys and produced another strong showing in the Boxing Day win over the Scarlets, the Dragons are able to offer a greater likelihood of regular first-team rugby in his preferred position.
Lewis Evans and Nic Cudd have shared the No7 duties for the Dragons in the Amlin Challenge Cup in Pool 2 but Lewis' current form and scavenging nature could add a different dimension next season.
The Dragons were beaten by Cardiff Blues at the Arms Park on New Years Day but are the second-best placed Welsh region in the RaboDirect PRO12 table, with Lewis' current club out in front.
They return to Amlin Challenge Cup action a week on Friday knowing that nothing but a win, and a win in which Bath don't pick up a bonus point of any sort, will see their hopes of quarter-final qualification remain ahead of Round 6.