The actress, who previously appeared in Death Comes To Pemberley and The White Queen, will star as Demelza, daughter of an impoverished and brutal miner who is rescued from a life of violence and drudgery by Ross Poldark.
Turner’s casting in the new version of Winston Graham’s acclaimed 18th century saga was confirmed last month.
A 1996 ITV adaption of the saga failed to capture audiences, partly because of the absence of original stars Robin Ellis (Ross) and Angharad Rees (Demelza).
Talking about her casting, Tomlinson said: “I am conscious of the fact that the original series had a huge following and was tremendously popular.
“All I can promise is that I will do my utmost to do justice to the spirit of Winston Graham’s iconic character. Demelza was so brilliantly portrayed by Angharad Rees and I will do my very best to try not to disappoint the army of Poldark fans out there.”
The BBC has also announced the casting of Heida Reed as Elizabeth, promised to Ross as a young girl but who marries his cousin Francis amid rumours of his death abroad, and Ruby Bentall as Ross’s cousin and dutiful daughter to his Uncle Charles.
Ben Stephenson, Controller, BBC Drama Commissioning, says: “The wonderful combination of our talented and stellar cast and Debbie Horsfield’s well crafted characters will ensure Poldark, our new epic and romantic drama, will be a real treat for the BBC One audience.”