Rumours began after filming was scheduled to take place at EUE/Screen Gems Studios in Wilmington, North Carolina. The studios are known for their special-effects water tank, ideal for filming underwater scenes featuring the King of Atlantis.
According to ComicBook.com, it is also thought that the Robert Downey Jr-starring film may be based on the classic Iron Man story 'Demon in a Bottle', which features the Sub-Mariner heavily.
Iron Man 2 contained various easter eggs which are thought to relate to Atlantis, as well as introducing Justin Hammer, another key player in 'Demon in a Bottle'.
Reports of a giant sea creature called Leviathan appearing in Joss Whedon's Avengers movie have also led to speculation that Atlantis and Namor may also be involved in that film.
The start date for shooting on Iron Man 3 was recently brought forward to May 4, with Captain America 2 planned to begin filming before the end of the year.
Other villains that have been rumoured to feature in Iron Man 3 are The Mandarin and The Ghost.