The administration would rule Kurdish-majority areas in the northeast until the broader conflict is over.
Kurdish groups in northern Syria have tried to stay out of the civil war between the government and rebels.
However, Islamist rebels have moved into Kurdish-controlled areas in recent months, causing increasing clashes between them and Kurdish militias.
The Kurdish Democratic Union party (PYD) announced the move after two days of talks in the town of Qamishli in north-eastern Syria.
Alan Semo, the PYD's UK-based representative, told the BBC his party was not seeking to carve out a separate Kurdish state.
Mr Semo said the region would be "integrated in future in a united, democratic, plural Syria", adding that neighbouring Turkey, which has fought a long war against Kurdish rebels, had nothing to fear from the move.
"I think Turkey will realise that this step is not a threat to any regional or international or global stability, it is temporary," he added.
The PYD planned to introduce a constitution and regional parliament, with representation for non-Kurds in the area, he went on.
However, it is not clear whether all Kurdish factions will take part in the administration.
"I think that such a rushed, one-sided move will be seen as... an obstacle to the Syrian opposition," Nuri Brimo, a member of another Kurdish grouping, the Kurdish National Council (KNC), told AFP.
A de facto autonomous Kurdish zone was created when government forces withdrew from the area in the summer of 2012.
Last month Kurdish fighters took control of a border crossing with Iraq from an Islamist rebel group.
Syria's Kurdish minority makes up around 10% of the population and is largely concentrated in the country's far north-east, near the borders with Turkey and Iraq.
The fighting between jihadist rebels and armed Kurdish militia has added another layer to Syria's complex civil war.
