He was arrested in December 2013 after accusations that he withheld billions in oil revenue channelled through his companies. He denies the allegations.
Zanjani was convicted of fraud and economic crimes, a judiciary spokesperson said at a press briefing.
Two others were also sentenced to death and all were ordered to repay embezzled funds.
One of Iran's richest men, Zanjani had been blacklisted by the US and EU for helping Iran evade oil sanctions in place at the time.
He had acknowledged using a web of companies in the UAE, Turkey, and Malaysia to sell millions of barrels of Iranian oil on behalf of the government.
Mr Zanjani is said to be worth about some $13.5bn (9.5bn).
By BBC
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