Iranian media reported that Abdullah Ramezanzadeh was convicted of trying to topple the government during protests after elections last June.
The charges against him included "acts against the national security, propaganda against the Islamic state and holding classified documents".
Other opposition supporters have been sentenced to death by courts following the anti-government protests.
Mr Ramezanzadeh supported pro-reform candidate Mir Houssein Mosavi during the election.
He was a government spokesman under reformist President Mohammed Khatami between 1997 and 2005.
Clashes
Reports say more than 100 other people have been jailed since the protests over the polls.
Sentences handed out by the courts for journalists and activists arrested during the protests have been up to 15 years.
As many as five people have been sentenced to death, prosecutors say.
Thousands were arrested and dozens killed during the largest street protests since the 1979 Islamic Revolution.
They were sparked by opposition claims of election fraud in the presidential race.
Clashes broke out earlier this week between the government and pro-reform supporters following the death of reformist cleric Grand Ayatollah Hoseyn Ali Montazeri.
