The court decided unanimously on Wednesday to overturn the verdicts pronounced in April by a state security court against leading activist Leyla Zana and her co-defendants after a retrial.
The four had been granted a retrial after their original trial in 1994 was condemned as unfair by the European Court of Human Rights.
They were released last month pending appeal following the government's decision, as part of its EU-inspired reforms, to abolish the state security court system which had previously
been used to try political and security-related crimes.
"The decision of the Ankara First State Security Court sentencing ex-lawmakers Leyla Zana, Orhan Dogan, Hatip Dicle and Selim Sadak to 15 years imprisonment has been unanimously overturned," Judge Hasan Gerceker said in televised remarks.
The four former parliamentarians would have been released in any case next year, having served 10 years of their 15-year sentences. They will not have to return to jail during the retrial.
The politicians were due to
be released next year
In an additional complication, Turkish police said last week they had filed a complaint against the four for attending rallies and addressing them in Kurdish since their release.
It was not immediately clear whether this move would have any impact on the fate of the four ex-lawmakers.
Turkey's powerful security establishment views the four as subversives bent on undermining national unity. They were jailed at the height of an armed separatist campaign in south-eastern Turkey which claimed the lives of more than 30,000 people.
Fighting in the region largely subsided after the 1999 capture of rebel leader Abd Allah Ocalan but the guerrillas recently called off a six-year unilateral ceasefire and there has been an upturn in violent clashes with security forces.
The EU has described the four ex-lawmakers as "political prisoners" and made clear their continued detention could harm Turkey's hopes of persuading the bloc at a summit in December to open long-delayed entry negotiations.
Turkey, which for decades denied the very existence of its Kurdish minority, has recently eased restrictions on broadcasting and teaching in the Kurdish language.
Turkey has an estimated 12 million Kurds in a total population of 70 million.