The Council of Europe has repeatedly called on Turkey to immediately release the 65-year-old activist.
A Turkish court has upheld the life sentence of civil rights activist and philanthropist Osman Kavala and seven other defendants.
The eight suspects were all found guilty in April of "attempting to overthrow the government" and imprisoned for 18 years without parole.
President Recep Tayyip Erdogan has accused Kavala of financing mass anti-government protests in 2013. He has always denied the allegations.
The 65-year-old was arrested in 2017 and was detained for more than four years until his trial despite international criticism.
The Council of Europe has repeatedly called on Turkey to immediately release Kavala after a European court ruled that Ankara had violated his human rights.
Germany initially summoned Turkey’s ambassador to Berlin to protest against the conviction.
Kavala was initially acquitted in February 2020 of charges that connected him with the 2013 protests.
But, as supporters awaited his release, he was re-arrested on charges linked to a 2016 coup attempt, which the Turkish government has blamed on a network of US-based Muslim cleric Fethullah Gulen.
This acquittal was later overturned and the cases were merged.
The European Court of Human Rights said in 2019 that Kavala’s imprisonment aimed to silence him and other human rights defenders.
Kavala's conviction can still be appealed to the Turkish Court of Cassation.