President of the region Carles Puigdemont said Catalans will be asked to answer yes or no to a single question: “Do you want Catalonia to become an independent state in the form of a republic?”
The Spanish government in Madrid has said it will block any attempt to divorce the region from the rest of the country.
Prime minister Mariano Rajoy, has yet to comment on the announcement but has previously said he would not recognise the vote.
The Spanish government’s spokesman, Inigo Mendez de Vigo, said: “That referendum will not take place because it is illegal.
“We are facing an increasingly radical strategy that has less and less support.
“Any move that evolves from an announcement to a fact will be appealed by the government.”
Catalan voters were last asked for their views on independence in an informal ballot in 2014, which showed more than 80% supported leaving Spain, although turnout was low.