| Michael Rennie | .... | Klaatu |
| Patricia Neal | .... | Helen Benson |
| Hugh Marlowe | .... | Tom Stevens |
| Sam Jaffe | .... | Professor Jacob Barnhardt |
| Billy Gray | .... | Bobby Benson |
| Frances Bavier | .... | Mrs. Barley |
| Lock Martin | .... | Gort |
| Holly Bane | .... | Army Captain (uncredited) |
| Marshall Bradford | .... | Chief of Staff (uncredited) |
| John Brown | .... | Mr. George Barley (uncredited) |
| John Burton | .... | British Radio M.C (uncredited) |
| Wheaton Chambers | .... | Mr. Bleeker, Diamonds (uncredited) |
| John Close | .... | Captain (uncredited) |
| Frank Conroy | .... | Mr. Harley (uncredited) |
| Eric Corrie | .... | British Soldier (uncredited) |
| John Costello | .... | Cockney (uncredited) |
| James Craven | .... | Businessman (uncredited) |
| Marjorie Crossland | .... | Hilda (uncredited) |
| Jack Daly | .... | Bit Man (uncredited) |
| Elmer Davis | .... | Himself (uncredited) |
| Lawrence Dobkin | .... | Army physician (uncredited) |
| Jim Doyle | .... | A doctor (uncredited) |
| Charles Evans | .... | Major general (uncredited) |
| Edith Evanson | .... | Mrs. Crockett (uncredited) |
| Michael Ferris | .... | British Soldier (uncredited) |
| Elizabeth Flournoy | .... | Jewelry clerk (uncredited) |
| Grady Galloway | .... | American Radar Operator (uncredited) |
| Bill Gentry | .... | Sentry (uncredited) |
| Glenn Hardy | .... | Interviewer (uncredited) |
| Harry Harvey | .... | Taxi Driver (uncredited) |
| Gabriel Heatter | .... | Himself, Voice (uncredited) |
| Gil Herman | .... | Government agent (uncredited) |
| H. V. Kaltenborn | .... | Himself (uncredited) |
| Hassan Khayyam | .... | Indian Radio Announcer (uncredited) |
| Harry Lauter | .... | Platoon Leader (uncredited) |
| Freeman Lusk | .... | General Cutler (uncredited) |
| George Lynn | .... | Colonel Ryder (uncredited) |
| Herbert Lytton | .... | Brigadier General (uncredited) |
| Michael Mahoney | .... | Sentry (uncredited) |
| David McMahon | .... | Air Force sergeant (welder) (uncredited) |
| Tyler McVey | .... | Brady, Gvt. Agent (uncredited) |
| Millard Mitchell | .... | General Cutler's Voice (uncredited) |
| Howard Negley | .... | Colonel (uncredited) |
| Dorothy Neumann | .... | Margaret (uncredited) |
| Sammy Ogg | .... | Boy (uncredited) |
| Robert Osterloh | .... | Army physician (uncredited) |
| Gayle Pace | .... | Captain (uncredited) |
| Drew Pearson | .... | Himself (uncredited) |
| Ted Pearson | .... | Colonel (uncredited) |
| House Peters, Jr. | .... | MP Captain (uncredited) |
| John M. Reed | .... | Tank Driver (uncredited) |
| Fay Roope | .... | Major General (uncredited) |
| James Seay | .... | Government Man (uncredited) |
| Olan Soule | .... | Mr. Krull, Boarder (uncredited) |
| Kim Spalding | .... | Army Orderly (uncredited) |
| Harmon Stevens | .... | Bit Man (uncredited) |
| Harlan Warde | .... | Carson (uncredited) |
| Stuart Whitman | .... | Army sentry (uncredited) |
| Adam Williams | .... | Army Physician (uncredited) |
| Rush Williams | .... | M.P. Sergeant (uncredited) |
| Carleton Young | .... | Colonel (uncredited) |