Karroubi, 72, dubbed the "sheikh of reforms", is the third of the four leading prospective candidates who have registered for the vote.
Incumbent President Mahmoud Ahmadinejad on Friday declared his intention to seek a second term while the former head of the Revolutionary Guards Corps, Mohsen Rezai, has also signed on for the election.
Former Iranian prime minister Mir Hossien Mousavi is expected later Saturday to formally register his candidacy.