| 31 Aug 18 | G. Saville George Saville |  MillwallMIL | → |  MiddlesbroughMID | Loan |
| 31 Aug 18 | R. Leonard Ryan Leonard |  Sheffield UnitedSHU | → | MillwallMIL | Loan |
| 30 Aug 18 | F. Onyedinma Fred Onyedinma | MillwallMIL | → |  Wycombe WanderersWYC | Loan |
| 23 Aug 18 | T. Bradshaw Tom Bradshaw |  BarnsleyBAR | → | MillwallMIL | Loan |
| 17 Aug 18 | J. Karacan Jem Karacan | XXX | → | MillwallMIL | Free |
| 17 Aug 18 | B. Thompson Ben Thompson | MillwallMIL | → |  PortsmouthPOR | Loan |
| 02 Aug 18 | J. Skalak Jiri Skalak |  Brighton & Hove AlbionBHA | → | MillwallMIL | Undisclosed |
| 02 Aug 18 | J. Skalak Jiri Skalak | Brighton & Hove AlbionBHA | → | MillwallMIL | Undisclosed |
| 02 Aug 18 | S. Nelson Sid Nelson | MillwallMIL | → |  Swindon TownSWI | Loan |
| 13 Jul 18 | B. Amos Ben Amos |  Bolton WanderersBOL | → | MillwallMIL | Loan |
| 01 Jul 18 | Tom King | MillwallMIL | → |  AFC WimbledonWIM | Loan |
| 20 Jun 18 | M. Wallace Murray Wallace |  Scunthorpe UnitedSCU | → | MillwallMIL | Undisclosed |
| 31 Jan 18 | B. Marshall Ben Marshall |  Wolverhampton WanderersWOL | → | MillwallMIL | Loan |
| 29 Jan 18 | H. Toffolo Harry Toffolo |  Norwich CityNOR | → | MillwallMIL | Loan |
| 29 Jan 18 | H. Toffolo Harry Toffolo | Norwich CityNOR | → | MillwallMIL | Undisclosed |
| 29 Jan 18 | Tim Cahill | XXX | → | MillwallMIL | Free |
| 26 Jan 18 | K. Twardek Kristopher Twardek | MillwallMIL | → |  Carlisle UnitedCAR | Loan |
| 25 Jan 18 | J. Shackell Jason Shackell |  Derby CountyDER | → | MillwallMIL | Loan |
| 24 Jan 18 | P. Rooney Paul Rooney | MillwallMIL | → |  Colchester UnitedCOL | Undisclosed |
| 04 Jan 18 | T. King Tom King | MillwallMIL | → |  StevenageSTE | Loan |
| 04 Jan 18 | S. Nelson Sid Nelson | MillwallMIL | → |  ChesterfieldCHES | Loan |