CHP lawmaker Enis Berberoglu had not been detained during his trial but after the verdict was announced he was immediately placed under arrest at the Istanbul criminal court, state-run Anadolu news agency said.
A dozen lawmakers from the pro-Kurdish Peoples' Democratic Party (HDP) have been arrested under the state of emergency that followed last year's failed coup.
But this is the first time a deputy from the CHP -- the main secular opposition founded by Turkey's first president Mustafa Kemal Ataturk -- has been held.
Cumhuriyet's former editor in chief, Can Dundar, was last year handed a five-year-and-10-month jail term in the case. He has since fled to Germany.
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Berberoglu was accused of handing over the images to Dundar. He was given a life sentence but this was reduced to 25 years by the court.
The CHP immediately held an emergency meeting after the verdict and its leader Kemal Kilicdaroglu was due to make a statement.