Carola Rackete, a 31-year-old German national, disobeyed Italian military orders and entered the port of Lampedusa on Saturday, hitting a police patrol boat as she brought some 41 African migrants to land in the Sea-Watch rescue vessel.
She was immediately detained and placed under house arrest, but in a blow for Italy's hardline interior minister, Matteo Salvini, Judge Alessandra Vella ruled that Rackete had been carrying out her duty and had not committed any act of violence.
Rackete still faces possible charges of helping illegal immigration, but Vella ordered her immediate release.
Salvini said in a statement he had hoped for a tougher response from the Italian justice system and promised to expel Rackete as soon as possible.
The controversy over Rackete's actions have dented relations between Italy on the one side and Germany and France on the other, and have highlighted Europe's continued failure to adopt coherent immigration norms.
Rackete appeared before the Agrigento court on Monday and apologised for hitting the patrol boat, saying it had been an accident and explaining that her sole concern was the well-being of the migrants who had been at sea for more than two weeks.
