But the detentions have rekindled concerns about freedom of expression in Turkey, analysts say.
The 15 released are lecturers at Kocaeli University in north-western Turkey, Anadolu reported. The three others are from Uludag University in Bursa province.
But the number of academics arrested could be higher, the Reuters news agency reported.
The declaration, signed by more than 1,000 academics in Turkey and abroad, calls on the government to halt "massacres" in the south-east.
The signatories stated that they refused to be "a party to the crime" and called for the resumption of peace efforts with the rebels.
The document was prompted by a Turkish government offensive against the outlawed separatist group Kurdistan Workers` Party, or PKK, that includes military operations and curfews.
But President Erdogan accused those who signed the document - including renowned international linguist and philosopher Noam Chomsky - of engaging in "terrorist propaganda" on behalf of the PKK.
The PKK is considered a terrorist group by Turkey and its Western allies and has fought a 30-year separatist insurgency in south-eastern Turkey that has killed tens of thousands of people.
Mr Erdogan accused the academics of being biased against the state and denounced them for failing to condemn rebel violence. He urged the judiciary to take action against their "treachery".
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