| Emily Watson | Ulana Khomyuk |
| Matthew Needham | Dmitri |
| Nadia Clifford | Dr. Svetlana Zinchenko |
| Rosie Sheehy | Nurse Pripyat |
| Sam Troughton | Alexandr Akimov |
| Robert Emms | Leonid Toptunov |
| Karl Davies | Viktor Proskuryakov |
| Ron Cook | Old Maternity Doctor |
| Adam Nagaitis | Vasily Ignatenko |
| Jessie Buckley | Lyudmilla Ignatenko |
| Tomas Zaibus | Soldier (Hospital) |
| Jared Harris | Valery Legasov |
| Amanda Drew | Kremlin Aide Female |
| Stellan Skarsgård | Boris Shcherbina |
| David Dencik | Mikhail Gorbachev |
| Alan Williams | KGB Chairman Charkov |
| Michael Socha | Mikhail |
| Laura Elphinstone | Oksana |
| Jan Ricica | Oksana's Kid |
| Peter Guinness | Major Burov |
| Laurence Spellman | Commission Heli Pilot |
| Alison Pargeter | BCP HQ Aide Female |
| Victor McGuire | Garanin |
| Sakalas Uzdavinys | Ilya Ivanovich Chulkov |
| Adrian Rawlins | Nikolai Fomin |
| Con O'Neill | Viktor Bryukhanov |
| Mark Lewis Jones | General Pikalov |
| Aidas Jurgaitis | Soldier (Shelter) |
| Aiste S. Gram | Bartender in Polyssia Hotel
(as Aistè Gramantaitè) |
| Cait Davis | Local Wife at Bar |
| Darius Petkevicius | Local Husband |
| Daniel Crossley | Stasiuk |
| Lucy Russell | Marina Gruzinskaya |
| Donald Sumpter | Zharkov |
| Diarmaid Murtagh | Zukauskas |
| Natasha Radski | Russian News Reader |
| Yitzchak Averbuch | General |
| Joe Tucker | Plant Employee |
| Baltasar Breki Samper | Ananenko |
| Philip Barantini | Bezpalov |
| Oscar Dyekjær Giese | Baranov
(as Oscar Giese) |
| Kadrolsha Ona Carole | Civilian in a Hospital
(uncredited) |
| Bartas Erminas | Civilian in a Hospital
(uncredited) |
| Peter Jennings | Self
(archive footage) (uncredited) |