The news comes weeks after Sons of Anarchy's Charlie Hunnam dropped out of the role over time constraints, though some media reports speculated that the 33-year-old actor didn't gel with the film's creative team and "got cold feet" about the intense media scrutiny.
Dornan, 31, has previously played Once Upon a Time's ridiculously good-looking, hipster-inspired Sheriff Graham. He currently stars in the U.K. series The Fall.
The Hollywood Reporter also said that Dornan did a screen test with Dakota Johnson in front of director Sam Taylor-Johnson, 50 Shades author E.L. James and the film's producers.
The film is slated for an August 2014 release date.
UPDATE: Here's how James, 50 Shades' social-media savvy author, responded to the news:
I'm a monstrous @JamieDornan1 fan. Wasn't allowed to be attracted to him on The Fall bc he played a sexmurderer. 50 Shades is my big chance!— Lena Dunham (@lenadunham) October 24, 2013
Jamie Dornan as Christian Grey. Hmm he's okay but I was still hoping for Matt Bomer tho— Jabe Daud (@jaaaaabe) October 24, 2013
JAMIE DORNAN IS SHERIFF GRAHAM FROM ONCE UPON A TIME YESSSS LORD OMG ROSA PARK SAT UPFRONT FOR THIS— jimmy neutron (@biebero2lbrien) October 24, 2013
Here's an example of the smoldering skills Dornan will bring to his work on 50 Shades: