Also veteran stage star Patti LuPone has hopped along for the ride from series creator Shawn Ryan.
The actresses join a cast that already includes Jessica Lange, Kathy Bates, Evan Peters, Sarah Paulson, Taissa Farmiga, Frances Conroy and Lily Rabe.
The new season arrives on October 9 at 10 PM ET/PT on FX.
Here's a breakdown if you missed it previously:
American Horror Story: Coven tells the secret history of witches and witchcraft in America. Over three hundred years have passed since the turbulent days of Salem and those who managed to escape are now facing extinction. Mysterious attacks have been escalating against their kind and young girls are being sent away to a special school in New Orleans to learn how to protect themselves. Wrapped up in the turmoil is new arrival, Zoe, who is harboring a terrifying secret of her own. Alarmed by the recent aggression, Fiona, the long-absent Supreme, sweeps back into town, determined to protect the Coven and hell-bent on decimating anyone who gets in her way.
The last time Bassett appeared as a series regular on any TV show was during the final season (2008–2009) of the once hit medical drama ER on NBC.
Sidibe was most recently a regular on Showtime's The Big C.
Ahead of the broadcast premiere of Coven, FX has been releasing short teasers, photos and posters, each one somewhat cryptic, unveiling bits and pieces of the new season. Here's another, which ends with Bassett's maniacal laugh. Underneath you'll find a new piece of poster art, also featuring Bassett: