Iran's Supreme Court has upheld the death sentence of Iranian-German citizen Jamshid Sharmahd on charges of "corruption on Earth", the judiciary said on Wednesday.
Berlin immediately condemned what it said was an unacceptable end to an unfair trial.
Mr Sharmahd, who also has US residency, is accused of being the mastermind of a 2008 mosque bombing that killed 14 people and wounded more than 200.
The sentence handed down in February "has been confirmed by the Supreme Court," said Iranian judiciary spokesman Masoud Setayeshi in comments carried by the judiciary's Mizan news agency.
"After the lower court is notified, actions will subsequently be taken to implement the Supreme Court's decision."
The German ambassador in Tehran broke off a business trip to protest to Iranian authorities, Foreign Minister Annalena Baerbock said.
"At no time did Jamshid Sharmahd have anything approaching a fair trial," Ms Baerbock said.
"We demand that Iran immediately reverses this arbitrary verdict."
German opposition leader Friedrich Merz said the news was "deeply shocking".
"I once again call on the regime in Iran to immediately allow Jamshid Sharmahd to leave for home in Germany," he said.
Norbert Roettgen, a conservative MP and former chair of the German parliament's foreign affairs committee, said the sentence against Mr Sharmahd could be carried out at any moment.
"His life is in acute danger," Mr Roettgen said.
"The kidnapping and political hostage-taking of Jamshid Sharmahd show the deeply inhumane way this regime behaves."
Dozens of dual and foreign citizens are being held in Iran, some of whom have been detained for more than a year with no formal charges pressed.
Relatives and rights groups have accused Tehran of using people as "bargaining chips" against western powers.
Mr Sharmahd's arrest was announced in 2020 by the Ministry of Intelligence, which accused him of leading a "terror ring" planning attacks on Iran from the US.
The family of Mr Sharmahd, who holds US residency, have told The National US President Joe Biden is "failing" them by distancing himself from the case.
Mr Sharmahd's daughter says he has shown signs of torture, including missing teeth, facial bruising and difficulty walking.
He was charged with "corruption on earth", a broad accusation that has been used against minority activists and protesters.
The sentence led to tit-for-tat expulsions of diplomats from Berlin and Tehran.
Germany has also condemned Iran for its treatment of protesters during unrest that broke out last autumn.
Berlin's Foreign Ministry said on Tuesday it was "maintaining pressure on Iran" by supporting new EU sanctions on the regime.
Targets of the latest EU sanctions include mobile network provider Ariantel, which Germany accused of carrying out surveillance against anti-regime voices.
Also on Wednesday, Belgium refuted claims from Tehran that a prisoner swap deal had been reached to free jailed aid worker Olivier Vandecasteele.
A representative for Belgian Justice Minister Vincent van Quickenborne denied the claims from Mr Setayeshi and lambasted a "false message from a rogue state that specialises in making false statements".
"They do this to manipulate and confuse an innocent compatriot and his family," he added.
Last week, Brussels requested a transfer for Mr Vandecasteele, who was sentenced to more than 12 years in jail for alleged spying, money laundering and smuggling currency.
Belgium's High Court upheld a prisoner exchange treaty with Tehran last month amid hopes of bringing the former aid worker home from Iran, where he has been held in solitary confinement since his arrest last year.
His health has deteriorated since his detention and friends say he has had regular infections and dental problems.
