Wenham's online filmography claims that she will appear in the BBC fantasy drama's fifth series.
A version of the Mab character previously appeared in Sam Neill's 1998 TV mini-series Merlin, in which she was depicted as the evil twin sister of the Lady of the Lake.
Wenham's past credits include ITV drama Where the Heart Is and Charlie Brooker's E4 zombie thriller Dead Set.
Guest stars confirmed for the next series of Merlin include Game of Thrones stars Liam Cunningham and John Bradley, as well as returning actors Lindsay Duncan (Queen Annis) and Janet Montgomery (Princess Mithian).
Merlin star Colin Morgan previously told Digital Spy that his young wizard character is in a "ruthless mode" in the next series.
"Merlin is becoming more ruthless, because he is becoming more solid in his beliefs," said the 26-year-old. "He's seeing things in a whole new way."
Merlin will return to BBC One later in the year.
> Merlin exec on shock return, promises 'amazing' scenes
> Merlin execs: 'The show needed to grow up a little bit'
Watch the stars of Merlin talk to DS below: