Michael Rennie
as Klaatu, alias Mr. Carpenter

Patricia Neal
as Helen Benson

Billy Gray
as Bobby Benson

Sam Jaffe
as Prof. Jacob Barnhardt

Hugh Marlowe
as Tom Stevens

Lock Martin
as Gort

Freeman Lusk
as General Cutler

Edith Evanson
as Mrs. Crockett, landlady

Frank Conroy
as Mr. Harley, Secretary to the President

Frances Bavier
as Mrs. Barley, boarder

John Brown
as George Barley, boarder

Olan Soule
as Mr. Krull, boarder

Marjorie Crossland
as Hilda, Barnhart's secretary

Elmer Davis
as Elmer Davis

H.V. Kaltenborn
as H. V. Kaltenborn

Drew Pearson
as Drew Pearson

Gabriel Heatter
as Gabriel Heatter (voice)

Harry Lauter
as Lieutenant in Charge of Landing Site

James Doyle
as Army Dr. White

Larry Dobkin
as Balding Army Doctor

Robert Osterloh
as Army Examining Doctor

Glenn Hardy
as Radio Interviewer at Landing Site

Tyler McVey
as Brady

House Peters Jr.
as M.P. Captain at Barnhardt's

George Lynn
as Conference Colonel Ryder

Dorothy Neumann
as Margaret, Tom's secretary

Wheaton Chambers
as Mr. Bleeker, jeweller

Carleton Young
as Zone Five Lieutenant Colonel

Harry Harvey
as The Cab Driver

Sammy Ogg
as Sammy, boy witness

Bess Flowers
as Lady Outside Jewelry Store

James Seay
as Government Man

Grady Galloway
as Radar Operator

Hassan Khayyam
as Indian Newscaster

John Barton
as British Newscaster

Stuart Whitman
as Sentry (scene deleted)

Rama Bai
as Scientific Delegate

John Burton
as British Radio Announcer

Bill Welsh
as Radio Announcer (voice)