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)