She will take the role of Carmel Kazemi, Kush's mother.
She will start filming her scenes this month and will make her first screen appearance in June.
Known for her role as Mel Bush, Doctor Who's assistant as well as her stage roles in Chicago and Cats, Langford said: "I'm so thrilled and delighted to be part of EastEnders."
She added: "I'm a great fan of the show and think the recent 30th anniversary was sensational and shows just how good British television can be. To be part of this family is an absolute privilege."
Dominic Treadwell-Collins, executive producer for EastEnders, described Langford as a "British institution" as well as an "old-school hard-graft actress gifted at both comedy and serious drama".
He added: "Bonnie brings wit, pathos and a touch of class to Carmel Kazemi - a thoroughly modern Essex divorcee who isn't quite ready to cut the apron strings firmly attached to her son Kush.
"Even if it means clashing spectacularly with an already prickly Shabnam."
Theatre actress and 70s child star Bonnie Langford is joining the cast of EastEnders, the BBC says.