| 31 Aug 21 | C. Coventry Conor Coventry |  West Ham UnitedWHU | → |  Peterborough UnitedPET | Loan |
| 31 Aug 21 | X. Silva Xande Silva | West Ham UnitedWHU | → |  Nottingham ForestFOR | Undisclosed |
| 31 Aug 21 | N. Vlasic Nikola Vlasic |  CSKA MoscowCSK | → | West Ham UnitedWHU | Undisclosed |
| 31 Aug 21 | C. Coventry Conor Coventry | West Ham UnitedWHU | → | Peterborough UnitedPET | Loan |
| 31 Aug 21 | A. Král Alex Král |  Spartak MoscowSPM | → | West Ham UnitedWHU | Loan |
| 31 Aug 21 | X. Silva Xande Silva | West Ham UnitedWHU | → | Nottingham ForestFOR | Undisclosed |
| 30 Aug 21 | A. Odubeko Ademipo Odubeko | West Ham UnitedWHU | → |  Huddersfield TownHUD | Loan |
| 30 Aug 21 | A. Odubeko Ademipo Odubeko | West Ham UnitedWHU | → | Huddersfield TownHUD | Loan |
| 28 Aug 21 | K. Zouma Kurt Zouma |  ChelseaCHE | → | West Ham UnitedWHU | Undisclosed |
| 13 Aug 21 | F. Alves Frederik Alves | West Ham UnitedWHU | → |  SunderlandSUN | Loan |
| 29 Jul 21 | N. Holland Nathan Holland | West Ham UnitedWHU | → |  Oxford UnitedOXF | Loan |
| 29 Jul 21 | A. Areola Alphonse Areola |  Paris Saint-Germain PSG | → | West Ham UnitedWHU | Loan |
| 29 Jul 21 | A. Areola Alphonse Areola | Paris Saint-Germain PSG | → | West Ham UnitedWHU | Loan |
| 16 Jul 21 | Felipe Anderson | West Ham UnitedWHU | → |  LazioLAZ | Undisclosed |
| 16 Jul 21 | Felipe Anderson | West Ham UnitedWHU | → | LazioLAZ | Undisclosed |
| 01 Jul 21 | C. Dawson Craig Dawson |  WatfordWAT | → | West Ham UnitedWHU | Undisclosed |
| 01 Jul 21 | C. Dawson Craig Dawson | WatfordWAT | → | West Ham UnitedWHU | Undisclosed |
| 01 Jul 21 | J. Anang Joseph Anang | West Ham UnitedWHU | → |  StevenageSTE | Loan |
| 01 Feb 21 | O. Afolayan Oladapo Afolayan | West Ham UnitedWHU | → |  Bolton WanderersBOL | Loan |
| 01 Feb 21 | W. Reid Winston Reid | West Ham UnitedWHU | → |  BrentfordBRE | Loan |
| 01 Feb 21 | W. Reid Winston Reid | West Ham UnitedWHU | → | BrentfordBRE | Loan |
| 01 Feb 21 | A. Alese Ajibola Alese | West Ham UnitedWHU | → |  Cambridge UnitedCAMB | Loan |
| 30 Jan 21 | G. Cardoso Gonçalo Cardoso | West Ham UnitedWHU | → |  FC BaselBAS | Loan |
| 29 Jan 21 | S. Benrahma Saïd Benrahma | BrentfordBRE | → | West Ham UnitedWHU | Undisclosed |
| 29 Jan 21 | S. Benrahma Saïd Benrahma | BrentfordBRE | → | West Ham UnitedWHU | Undisclosed |
| 29 Jan 21 | J. Lingard Jesse Lingard |  Manchester UnitedMAN | → | West Ham UnitedWHU | Loan |
| 15 Jan 21 | D. Kemp Daniel Kemp | West Ham UnitedWHU | → |  Leyton OrientLEY | Undisclosed |
| 08 Jan 21 | S. Haller Sébastien Haller | West Ham UnitedWHU | → |  Ajax AmsterdamAJA | Undisclosed |
| 08 Jan 21 | S. Haller Sébastien Haller | West Ham UnitedWHU | → | Ajax AmsterdamAJA | Undisclosed |
| 08 Jan 21 | R. Snodgrass Robert Snodgrass | West Ham UnitedWHU | → |  West Bromwich AlbionWBA | Undisclosed |
| 02 Jan 21 | F. Alves Frederik Alves | SIXSilkeborg IFSilkeborg IF | → | West Ham UnitedWHU | Undisclosed |