| 31 Aug 18 | G. Saville George Saville |  MillwallMIL | → |  MiddlesbroughMID | Loan |
| 31 Aug 18 | D. Batth Danny Batth |  Wolverhampton WanderersWOL | → | MiddlesbroughMID | Loan |
| 31 Aug 18 | D. Batth Danny Batth | Wolverhampton WanderersWOL | → | MiddlesbroughMID | Loan |
| 31 Aug 18 | M. Johnson Marvin Johnson | MiddlesbroughMID | → |  Sheffield UnitedSHU | Loan |
| 30 Aug 18 | S. McQueen Sam McQueen |  SouthamptonSOU | → | MiddlesbroughMID | Loan |
| 30 Aug 18 | S. McQueen Sam McQueen | SouthamptonSOU | → | MiddlesbroughMID | Loan |
| 30 Aug 18 | E. Mahmutovic Enes Mahmutovic | MiddlesbroughMID | → |  Yeovil TownYEO | Loan |
| 29 Aug 18 | N. McGinley Nathan McGinley | MiddlesbroughMID | → |  Forest Green RoversFGR | Loan |
| 23 Aug 18 | M. Besic Muhamed Besic |  EvertonEVE | → | MiddlesbroughMID | Loan |
| 23 Aug 18 | M. Besic Muhamed Besic | EvertonEVE | → | MiddlesbroughMID | Loan |
| 09 Aug 18 | C. Ripley Connor Ripley | MiddlesbroughMID | → |  Accrington StanleyACCR | Loan |
| 08 Aug 18 | J. Hugill Jordan Hugill |  West Ham UnitedWHU | → | MiddlesbroughMID | Loan |
| 08 Aug 18 | A. Traoré Adama Traoré | MiddlesbroughMID | → | Wolverhampton WanderersWOL | Undisclosed |
| 08 Aug 18 | A. Traoré Adama Traoré | MiddlesbroughMID | → | Wolverhampton WanderersWOL | Undisclosed |
| 08 Aug 18 | J. Hugill Jordan Hugill | West Ham UnitedWHU | → | MiddlesbroughMID | Loan |
| 05 Aug 18 | B. Gibson Ben Gibson | MiddlesbroughMID | → |  BurnleyBUR | Undisclosed |
| 05 Aug 18 | B. Gibson Ben Gibson | MiddlesbroughMID | → | BurnleyBUR | Undisclosed |
| 03 Aug 18 | A. Pattison Alex Pattison | MiddlesbroughMID | → | Yeovil TownYEO | Loan |
| 02 Aug 18 | A. Lonergan Andy Lonergan |  Leeds UnitedLEE | → | MiddlesbroughMID | Free |
| 31 Jul 18 | P. Bamford Patrick Bamford | MiddlesbroughMID | → | Leeds UnitedLEE | Undisclosed |
| 30 Jul 18 | C. Cooke Callum Cooke | MiddlesbroughMID | → |  Peterborough UnitedPET | Undisclosed |
| 18 Jul 18 | Fabio | MiddlesbroughMID | → |  NantesNANT | Undisclosed |
| 18 Jul 18 | Fabio | MiddlesbroughMID | → | NantesNANT | Undisclosed |
| 16 Jul 18 | G. Miller George Miller | MiddlesbroughMID | → |  Bradford CityBRD | Loan |
| 02 Jul 18 | A. Barragán Antonio Barragán | MiddlesbroughMID | → |  Real BetisBET | Undisclosed |
| 02 Jul 18 | A. Barragán Antonio Barragán | MiddlesbroughMID | → | Real BetisBET | Undisclosed |
| 01 Jul 18 | J. Fryer Joe Fryer | MiddlesbroughMID | → |  Carlisle UnitedCAR | Loan |
| 27 Jun 18 | A. Flint Aden Flint |  Bristol CityBRC | → | MiddlesbroughMID | Undisclosed |
| 26 Jun 18 | P. McNair Paddy McNair |  SunderlandSUN | → | MiddlesbroughMID | Undisclosed |
| 31 Jan 18 | M. Braithwaite Martin Braithwaite | MiddlesbroughMID | → |  BordeauxBOR | Loan |
| 31 Jan 18 | M. Besic Muhamed Besic | EvertonEVE | → | MiddlesbroughMID | Loan |
| 31 Jan 18 | M. Braithwaite Martin Braithwaite | MiddlesbroughMID | → | BordeauxBOR | Loan |
| 31 Jan 18 | M. Besic Muhamed Besic | EvertonEVE | → | MiddlesbroughMID | Loan |
| 31 Jan 18 | A. Guédioura Adlène Guédioura | MiddlesbroughMID | → |  Nottingham ForestFOR | Undisclosed |
| 31 Jan 18 | M. Cranie Martin Cranie |  Huddersfield TownHUD | → | MiddlesbroughMID | Undisclosed |
| 31 Jan 18 | C. Christie Cyrus Christie | MiddlesbroughMID | → |  FulhamFUL | Undisclosed |
| 31 Jan 18 | A. Fletcher Ashley Fletcher | MiddlesbroughMID | → | SunderlandSUN | Loan |
| 31 Jan 18 | M. Cranie Martin Cranie | Huddersfield TownHUD | → | MiddlesbroughMID | Undisclosed |
| 30 Jan 18 | J. Harrison Jack Harrison |  Manchester CityMNC | → | MiddlesbroughMID | Loan |
| 30 Jan 18 | J. Harrison Jack Harrison | Manchester CityMNC | → | MiddlesbroughMID | Loan |
| 19 Jan 18 | G. Miller George Miller | MiddlesbroughMID | → |  BuryBURY | Loan |
| 18 Jan 18 | A. Forshaw Adam Forshaw | MiddlesbroughMID | → | Leeds UnitedLEE | 4500000 |
| 17 Jan 18 | M. Tavernier Marcus Tavernier | MiddlesbroughMID | → |  Milton Keynes DonsMKD | Loan |
| 13 Jan 18 | N. McGinley Nathan McGinley | MiddlesbroughMID | → |  Wycombe WanderersWYC | Loan |
| 05 Jan 18 | L. Wing Lewis Wing | MiddlesbroughMID | → | Yeovil TownYEO | Loan |
| 05 Jan 18 | C. Johnson Callum Johnson | MiddlesbroughMID | → | Accrington StanleyACCR | Undisclosed |
| 04 Jan 18 | C. Ripley Connor Ripley | MiddlesbroughMID | → | BuryBURY | Loan |