Additions
| from | Name | Pos | Team | Team | ||
|---|---|---|---|---|---|---|
| 06/2021 |  | Vincent Lorenz | G-AN | |||
| 02/2021 | | Robert Glatzel | FW |  | Cardiff City | Cardiff |
| 01/2021 | | Danny da Costa | DF |  | Eintracht Frankfurt | E. Frankfurt |
| 01/2021 | | Dominik Kohr | MF | | Eintracht Frankfurt | E. Frankfurt |
| 01/2021 |  | Bo Svensson | MA |  | FC Liefering | FC Liefering |
| 01/2021 | | Babak Keyhanfar | AS-CO | | FC Liefering | FC Liefering |
| 01/2021 | | Tijan Njie | G-AN |  | 1. FSV Mainz 05 U19 | Mainz 05 U19 |
| 12/2020 | | Jan Siewert | MA | |||
| 11/2020 | | Patrick Kaniuth | AS-CO |  | 1. FSV Mainz 05 II (U16) U17 | Mainz 05 U16 U17 |
| 10/2020 |  | Kevin Stöger | MF | |||
| 08/2020 | | Luca Kilian | DF |  | SC Paderborn 07 | Paderborn |
| 08/2020 |  | Alexandru Maxim | MF |  | Gaziantep FK | Gaziantep |
| 07/2020 | | Marius Liesegang | GK |  | 1. FSV Mainz 05 II | Mainz 05 II |
| 07/2020 |  | Dimitri Lavalée | DF |  | Standard Liège | Standard Liège |
| 07/2020 |  | Gerrit Holtmann | MF | | SC Paderborn 07 | Paderborn |
| 07/2020 | | Niklas Tauer | MF | | 1. FSV Mainz 05 U19 | Mainz 05 U19 |
| 07/2020 |  | Issah Abass | FW |  | FC Utrecht | FC Utrecht |
| 07/2020 | | Marlon Mustapha | FW | | 1. FSV Mainz 05 U19 | Mainz 05 U19 |
| 07/2020 | | Paul Nebel | FW | | 1. FSV Mainz 05 U19 | Mainz 05 U19 |
| 07/2020 | | Aaron Seydel | FW |  | Jahn Regensburg | Regensburg |
Departures
| from | Name | Pos | Team | Team | ||
|---|---|---|---|---|---|---|
| 01/2021 |  | Dong-won Ji | FW |  | Eintracht Braunschweig | Braunschweig |
| 01/2021 |  | Jean-Philippe Mateta | FW |  | Crystal Palace | Cr. Palace |
| 01/2021 | | Dimitri Lavalée | DF |  | Sint-Truidense VV | STVV |
| 01/2021 |  | Aarón Martín | DF |  | Celta de Vigo | Celta de Vigo |
| 01/2021 | | Issah Abass | FW |  | FC Twente | FC Twente |
| 10/2020 | | Ridle Baku | MF |  | VfL Wolfsburg | Wolfsburg |
| 09/2020 | | Florian Müller | GK |  | SC Freiburg | Freiburg |
| 08/2020 | | Gerrit Holtmann | MF |  | VfL Bochum | Bochum |
| 08/2020 | | Alexandru Maxim | MF | | Gaziantep FK | Gaziantep |
| 08/2020 | | Jonathan Meier | DF |  | Dynamo Dresden | Dresden |
| 08/2020 | | Aaron Seydel | FW |  | SV Darmstadt 98 | Darmstadt |
| 07/2020 | | Ronaël Pierre-Gabriel | DF |  | Stade Brestois 29 | Stade Brest |
| 07/2020 |  | Jeffrey Bruma | DF | | VfL Wolfsburg | Wolfsburg |
| 07/2020 | | Niklas Kölle | DF |  | TSG Hoffenheim II | Hoffenheim II |
| 07/2020 |  | Erkan Eyibil | MF | | 1. FSV Mainz 05 II | Mainz 05 II |
| 07/2020 |  | Taiwo Awoniyi | FW |  | Liverpool FC U21 | LFC U21 |