The announcement on Friday set the stage for months of confrontation with the central government in the capital, Madrid, which says such a vote is illegal and must not take place.
"The question will be: 'Do you want Catalonia to become an independent state in the form of a republic'," Puigdemont said in a televised statement after a meeting of his cabinet.
He said attempts to agree on a date and the wording of the question with the Madrid government - headed by Prime Minister Mariano Rajoy - failed and left him with no other choice than moving unilaterally.
"We have always made very diverse offers and all of them have been rejected without any exception," Puigdemont said.
Pro-independence campaigners staged a symbolic ballot, organised by volunteers rather than government officials to get around court restrictions, in 2014, months after Scots voted to stay in the UK.
Some two million people voted in favour of secession in that non-binding ballot, though turnaround was relatively low.
In March, Puigdemont's predecessor Artur Mas was banned from holding office for two years after organising the 2014 referendum.
It is not clear how far the legal wrangling may go this time as the Catalan regional government has said it would throw all its weight behind the vote.
Under the Spanish constitution, referendums on sovereignty must be held nationally, not regionally.
Under Article 155, Madrid has the power to intervene directly in the running of Catalonia's regional government, forcing it to drop the vote.
This could involve sending in the police or suspending the regional government's authority to rule.
This is widely seen as a last resort move, however, and many analysts believe the clash will instead culminate in regional elections in Catalonia.
Source: News agencies