Director and co-screenwriter Adam McKay made the announcement on his Facebook and Twitter.
Wiig joins Will Ferrell, who's returning as the unintentionally hilarious newsman Ron Burgundy for the follow-up to 2004's hit comedy.
Christina Applegate, Paul Rudd, David Koechner and Steve Carrell are already slated to star in the anticipated sequel.
"Anchorman: The Legend Continues" is slated to open December 20.