"I am absolutely thrilled to be playing Charlotte Heywood," Williams said in a statement Friday. "This role is such a blessing. She's a brilliant character -- modern, headstrong, with heart and a voice."
The costume drama is set to air on ITV in Britain and via PBS' Masterpiece in the United States.
Filming is underway in and around Bristol, England.
Screenwriter Andrew Davies -- whose credits include War & Peace, Les Misérables, and Pride and Prejudice -- is penning the screenplay for Sanditon.
Austen died in 1817 while she was working on the book. She is the author of the classic romances Emma, Pride and Prejudice, Sense & Sensibility, Mansfield Park, Persuasion and Northanger Abbey, all of which have been adapted multiple times for television and film.
"I'm very excited that we are bringing the world of Sanditon to the TV audience with such a brilliant ensemble cast, headed by star of the future Rose Williams as our heroine, independent and forthright Charlotte Heywood, together with Theo James as Sidney Parker, our Regency entrepreneur with an aura of danger. It's been such fun to develop Jane Austen's fragment into a series -- now I'm eager to see our exceptional cast bring Sanditon to life," Davies said.