The actor’s involvement was confirmed just days before filming is due to commence on the 8-part drama which stars Aidan Turner as Ross and Eleanor Tomlinson as Demelza.
Also joining the cast are Jack Farthing as ambitious banker George Warleggan, Kyle Solder as Ross’ cousin Francis Poldark, Warren Clarke as Ross’s uncle Charles Poldark, Phil Davies as Ross’s lazy manservant Jud and Beatie Edney as Jud’s wife Prudie.
Last month the BBC confirmed the casting of Heida Reed as Ross’s first love Elizabeth and Ruby Bentall as his cousin Verity.
Filming will take place across Cornwall and Bristol over the coming months and the series will transmit in 2015 on BBC One.
Executive producer Karen Thrussell said: “We’re so thrilled by the Poldark cast, and we feel particularly privileged that Robin Ellis has agreed to join this stellar line-up – it’s a great tribute to Debbie Horsfield’s scripts.
“Robin is looking forward to returning to Winston Graham’s world, and we will be welcoming him with open arms! We can’t wait to start shooting next week and begin this epic journey in the stunning Cornish landscape.”