Schily: The meeting revealed some "interesting details"
An official statement said the driver of a fuel lorry which blew up outside the Ghriba synagogue on the resort island of Djerba was responsible.
It was "a premeditated crime perpetrated by a Tunisian, Nizar Ben Mohamed Nasr Nawar, with the aid of one of his relatives resident in Tunisia," the statement read.
Nawar, a 24-year-old resident of the French city of Lyon, is believed to have died in the blast.
The announcement came after a meeting between German Interior Minister Otto Schily and Tunisian President Zine al-Abidine Ben Ali.
Mr Schily said that the two countries were now "100% sure it was a terrorist crime".
Having initially said the attack was an accident, Tunisia last week acknowledged that the 11 April blast may have been intentional.
Al-Qaeda claim
Mr Schily said he learned some "extremely interesting details" during his meeting with Mr Ben Ali.
The nature of the explosion, the type of fuel and the way the tanks were mounted on the lorry had all combined to convince investigators that the attack was deliberate, he said.
German ministers have said there is evidence that al-Qaeda was behind the attack, but Mr Schily said it was too soon to link it Osama Bin Laden's terrorist network.
"For the time being, we can't say it was al-Qaeda," he said.
The London-based Arab newspaper, Al-Quds Al-Arabi, has published a letter purporting to be from al-Qaeda, saying it had attacked the synagogue.
And French police sources have said that the writer's name matches that of the truck's driver, who also died in the blast.
Earlier in his trip, Mr Schily visited the scene of the explosion, laying flowers and observing a two-minute silence in memory of the victims.