The defendants were arrested in 2011 after the authorities uncovered what they said was a plot hatched by Tehran targeting the Bahrain Interior Ministry and the Saudi Arabia Embassy in Manama.
Bahrain's Sunni-led government has been facing mounting opposition from the majority Shiite population since 2011 with widespread demonstrations and calls for the ouster of the Sunni monarchy.
The Bahrain monarchy has often accused Shiite Iran of inciting hatred and sectarianism on the island, saying the ayatollahs seek to expand their control over the Arabian Peninsula using religion as a tool.
Bahrain News Agency said Monday three of those convicted were in contact with Iranian Revolutionary Guards and the Basij militia "to target sensitive and vital infrastructure facilities" in Bahrain.
BNA said Iran denied all conspiracy allegations.