The 2013 iOS/Android phone rerelease of Sonic 2 re-adds the once lost Hidden Palace level. It consists of one act, retains the unused dinosaur and bat Badniks with a few changes, introduces a new jellyfish Badnik and a new Robotnik boss at the end. The 2P Mystic Cave zone music plays in this level.

The entrance to Hidden Palace is found at the bottom of a once-infamous shaft in the Mystic Cave Zone. Originally the shaft had spikes at the bottom that could easily (and slowly) kill the player whether or not they were Super Sonic. The rerelease removes the spikes, thus falling into this hole now takes the player to Hidden Palace.

The 2013 iOS version of the game features a green Grounder that can be placed by using the debug mode. This is an homage to Grounder from Adventures of Sonic the Hedgehog.

There is an additional post-credits scene in the 2013 re-release that can be seen after collecting all of the emeralds that were missing in the original release. The scene shows the Death Egg crashing into Angel/Floating Island, and cuts to Robotnik angrily jumping on the "END" text, akin to Sonic the Hedgehog. Surrounding him are a number of bushes, through one of which Knuckles' eyes can be seen spying on Robotnik, as a foreshadowing towards Sonic the Hedgehog 3.

In Sonic 2 Remastered for iOS and Android, it's possible to access the original Hidden Palace Zone (from the prototype version of Sonic 2 for the Genesis), referred to as "Proto Palace Zone".

The level can be accessed by unlocking the level select screen and using the Sound Test to input a code, before selecting Hidden Palace Zone. If done correctly the player will be taken to Proto Palace Zone instead.