Secret stages are six optional stages in Sonic Forces. Unlike regular stages, secret stages first become available after the player reclaims a territory on the World Map. Finishing a secret stage will reward the player with extra Avatar items and offer bonus experience for the Avatar.
Secret stages have the same gameplay mechanics as regular stages in Sonic Forces and the same objectives (reaching the goal within a time limit). Secret stages however, are much shorter than regular stages regarding size and duration, being similar to the Challenge Acts from Sonic Generations and the smaller Acts in Sonic Colors. They are also always played from a 2.5D perspective and their gameplays put special emphasis on a particular gimmick or obstacles, such as flamethrowers or bomb-based blocks. The routes the player travels along in a secret stage are likewise different from those in regular stages, having been constructed from the ground up with additional plaforms and basic building blocks, such as Unbreakable Containers.
Each secret stage is unlocked after the player reclaims an area on the World Map (the stage select screen) from the Eggman Empire. This occurs whenever the player completes a specific stage. The reclaimed area determines the landscape of the respective secret stage. It is also worth noting that only Sonic the Hedgehog (as well as Super Sonic, provided the stage has been cleared at least once) and the Avatar are playable in the secret stages. Shadow the Hedgehog also becomes playable in the secret stages if the player has downloaded Episode Shadow.
On the World Map, the Stage Targets for secret stages look like round padlocks.