A statement by the prosecution said the court rejected the appeal by the five defendants.
The men were convicted in November of “spying for and seeking with Iran and its agents to carry out hostile acts against the kingdom”.
They were found guilty of working with Iran’s elite Revolutionary Guard to carry out attacks in Bahrain against public facilities and banks.
Two of them had received training in Iran in “the manufacture and use of explosives and firearms in preparation for carrying out these hostile attacks”, according to the charges.
The Gulf state has been shaken by unrest since it quelled a month-long, Shiite-led uprising demanding reforms in 2011.