Demirtas, the former leader and presidential candidate of the pro-Kurdish People's Democratic Party (HDP), has been in jail since 2016, when a crackdown on the group intensified.
He faces multiple trials including his main case, in which he could be sentenced to 142 years if convicted on terror charges. He denies the charges as politically motivated.
In his latest hearing, a court in the capital Ankara sentenced Demirtas to two years and six months for making statements considered threatening against a prosecutor, the HDP said in a statement.
Last year Demirtas had said the prosecutor would be held "accountable before the law."
Before his conviction, he told the court on Friday that "no crime has been committed," according to a party transcript.
Demirtas received a 3.5-year sentence earlier this year for insulting President Recep Tayyip Erdogan and was convicted of disseminating terror propaganda in 2018.
The 48-year-old led the HDP to its first-ever appearance in the Turkish parliament in 2015 and challenged incumbent Erdogan in the 2014 and 2018 presidential elections, AFP reported.
The European Court of Human Rights in March this year urged the immediate release of Demirtas, accusing Turkey of violating its previous rulings by failing to free him.
Turkey accuses the HDP of being a political front for the outlawed Kurdistan Workers' Party (PKK), which has waged an insurgency against the Turkish state since 1984.
The PKK is blacklisted as a terrorist organization by Ankara and its Western allies.
The HDP and Demirtas say they are targeted because they dare to oppose Erdogan.
Demirtas and another former party chief, Figen Yuksekdag, were among a dozen HDP MPs detained in November 2016 as part of a crackdown that followed an attempted overthrow of Erdogan.
The pressure on the HDP continues to this day, with frequent police raids against party members across the country.
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