- Michael Rennie as Klaatu
- Patricia Neal as Helen Benson[a]
- Hugh Marlowe as Tom Stevens
- Sam Jaffe as Professor Jacob Barnhardt[b]
- Billy Gray as Bobby Benson
- Frances Bavier as Mrs. Barley
- Lock Martin as Gort
| Guy Williams | Radio operator: "Holy mackerel, call headquarters, get the lieutenant." |
| Kenneth Kendall | BBC news presenter |
| Elmer Davis | "This is Elmer Davis again. We still don't know what it is or where it comes from ..." |
| Harry Harvey Sr. | Taxi driver listening to Elmer Davis on radio |
| Charles Tannen [voice only] | Radio newscaster: "We interrupt this program to give you a bulletin just received ..." |
| H. V. Kaltenborn | "This is H. V. Kaltenborn speaking. Here in the nation's capital there is anxiety and concern ..." |
| Louis Jean Heydt | Airforce captain anxiously looking upward at the approaching spaceship |
| Roy Engel | Government employee (in fedora) looking upward |
| Pat Aherne | General at Pentagon: "Get me the chief of staff" |
| Wilson Wood | Official: "Hello ... I want to speak to the President ..." |
| Drew Pearson | "Good afternoon, ladies and gentlemen, this is Drew Pearson. We bring you this special ..." |
| Harry Lauter | Platoon leader at spaceship landing site |
| Frank Conroy | Mr. Harley, secretary to the President: "I've been told you speak our language ..." |
| Fay Roope | Major general visiting the site of the spaceship landing: "Oh, Carlson, what's the report?" |
| Harlan Warde | Carlson, government metallurgist examining the spaceship: "No luck, sir. We've tried everything ..." |
| Stuart Whitman | Sentry guarding the spaceship |
| Robert Osterloh | Major White, Klaatu's physician at Walter Reed Hospital |
| Lawrence Dobkin | Army physician at Walter Reed Hospital examining Klaatu's X-ray |
| Edith Evanson | Mrs. Crockett, proprietor of boarding house where Klaatu rents a room |
| John Brown | Mr. Barley, husband of Mrs. Barley; resident at Mrs. Crockett's |
| Olan Soule | Mr. Krull, resident of Mrs. Crockett's boarding house |
| Gabriel Heatter [voice only] | "And now, on this Sunday morning, we ask some questions that have been haunting ..." |
| James Craven | Businessman among the crowd looking at the spaceship |
| Tyler McVey | Mr. Brady, government agent who comes to see Klaatu at the boarding house |
| House Peters Jr. | Military police captain who arrives with Klaatu at Professor Barnhardt's house |
| Franklyn Farnum | Man passing Klaatu and Helen Benson in her office building corridor |
| Wheaton Chambers | Mr. Bleeker, jeweler to whom Tom Stevens brings Klaatu's diamond for evaluation |
| Millard Mitchell [voice only] | General at Pentagon meeting |
| George Lynn | Colonel Ryder at Pentagon meeting who reports Gort was being encased in a block of KL93 plastic |
| Chet Brandenburg | Man unable to start his outboard motor due to the power outage |
| Carleton Young | Colonel in jeep riding towards Klaatu's boarding house: "Attention all units!" |
| Snub Pollard | Taxi driver listening to radio announcement |
| Spencer Chan | One of the world scientists gathered at Professor Barnhardt's meeting alongside the spaceship |
Top broadcast journalists of their time, Elmer Davis, H.V. Kaltenborn, Drew Pearson, and Gabriel Heatter, had cameo roles. Spencer Tracy and Claude Rains originally were considered for the part of Klaatu.[9][10]
