The pair will play Stephen Wraysford and Isabelle Azaire - two lovers who are torn apart by the
events of the First World War - in the adaptation of Sebastian Faulks's novel.
Redmayne, who played Angel Clare in the channel's adaptation of Tess of the D'Urbervilles, said: "Birdsong had an overwhelming impact on me when I first read it in my teens.
"It is a privilege to now be a part of bringing Faulks's masterpiece to the screen."
127 Hours actress Poésy, who rose to fame playing Fleur Delacour in the Harry Potter movies, said that she was "so happy to be a part of this exciting new adventure".
Abi Morgan, who is behind BBC Two's forthcoming 1950s newsroom drama The Hour, will write the scripts for the two-part adaptation.
Ben Stephenson, controller of BBC Drama commissioning, described Morgan's scripts as "sublime" and hailed the drama's leads as "an inspired young cast".
Filming begins next month in Budapest.