The letter, reported by the Wall Street Journal, urges Ayatollah Ali Khamenei toward a nuclear agreement.
Barack Obama stresses any co-operation on fighting ISIS is contingent on Iran reaching such an agreement by a November 24 diplomatic deadline.
The White House has declined to comment on Barack Obama’s “private correspondence”.
The letter, sent last month, is at least the fourth time Barack Obama has written to the Iranian leader since taking office in 2009 and underscores his view that Iran is important in an emerging campaign against ISIS.
Officials with the Obama administration have, in recent days, placed the chances for a deal on Iran’s nuclear program at only 50-50, according to the Wall Street Journal.
Secretary of State John Kerry is scheduled to begin negotiations on the issue with Iranian counterpart Javad Zarif this weekend in Oman.
World powers suspect Iran of trying to make a nuclear bomb, a claim it denies.
An interim deal agreed late last year gave Iran some relief from sanctions in return for curbs on nuclear activity.
However, talks later stalled on the extent of uranium enrichment Iran would be allowed and on the timetable for sanctions to be lifted.
On November 6, White House spokesman Josh Earnest declined to comment directly on the secret letter.
“I can tell you that the policy that the president and his administration have articulated about Iran remains unchanged,” he said in response to questions.
Also on November 6, Republican speaker of the House John Boehner said he did not trust Iran’s leaders and said they should not be brought into the fight against ISIS.
lr1i7kwZm_IImage source: Wikimedia Commons President-elect Donald Trump celebrated his election victory at the Ultimate Fighting…
Millions of voters across the US chose to return Donald Trump to the White House…
Donald Trump declares victory in the US election as he addresses jubilant supporters in Florida.…
Stocks around the world are rising as Donald Trump appears to be on the cusp…
Donald Trump has won Pennsylvania, North Carolina and Georgia and taken a lead over Kamala…
Quincy Jones, the celebrated musician and producer who worked with Michael Jackson, Frank Sinatra, Ray…