Ayatollah Mahmoud Hashemi Shahrudi also banned the publication of photographs or films of executions.
Human rights groups say Iran executed nearly 300 people in 2007 as it tries to improve security and reduce what it calls immoral behaviour.
Most executions take place inside prisons but some criminals have been hanged from cranes in public.
Correspondents say it appears Ayatollah Shahrudi wants to lower the profile of executions as Iran has been widely criticised by Western countries and international organisations.
Iran has executed at least 28 convicts so far this year, according to media reports.
Capital offences in Iran include murder, rape, armed robbery, serious drug trafficking, apostasy, adultery and male homosexuality.
