Marvel has announced the news of the new addition at the New York Comic-Con.
The the actress is tapped in a key role which is yet to be revealed, reported AceShowbiz.
Marvel TV's executive vice president Jeff Loeb said Weaver would portray the main villain on the show.
"Sigourney Weaver, what more is there to say. She's a formidable actress and we are thrilled to have her join our remarkable cast," said Cindy Holland, Vice President of Netflix.
"The Defenders" also consists of Charlie Cox's "Daredevil", Krysten Ritter's "Jessica Jones", Mike Colter's "Luke Cage" and Finn Jones' "Iron Fist".
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