The court found Mariam Naqqash guilty of "endangering national security by spreading religious propaganda in the country", the website reported.
Naqqash was also accused of spying for Britain and Israel and is being held in Tehran's Evin prison.
Rights activists report that numerous underground churches have sprung up in Tehran and the nearby cities amid a sharp increase in recent years in the number of young people converting to Christianity from Islam.
Conversions from Islam to Zoroastrianism and the Bahai faith have also been on the rise, according to the activists.
Iran's Sharia law does not allow conversations from Islam to other religions and renouncing the Muslim faith is punishable with the death penalty.
Over 300 Iranian converts to Christianity have been arrested over the past two years, according to opposition websites.
Conversions from Islam to other faiths often take place in secret.