Former Catalan leader Artur Mas has been banned from holding office for two years after organising an illegal independence referendum in 2014.
The 61-year-old was convicted in Catalonia’s Superior Court of Justice of civil disobedience for organising the symbolic, non-binding poll.
Spain’s Constitutional Court had banned the vote at the time.
Catalonia, a region in the country’s north-east, has long campaigned for greater autonomy.
The pro-independence Esquerra Republicana de Catalunya (ERC) party called the verdict “disgraceful” and “undemocratic”.
Mr Mas said: “We will appeal in Spain and then take the case to European courts, if we need to.”
The Catalonia court also fined Mr Mas, who was president of Catalonia from 2010 until last year, €36,500 (£32,000; $39,000).
Two former members of his government – his deputy Joana Ortega and education minister Irene Riga – were also found guilty and banned from public office for 21 and 18 months respectively. They were also fined.
Thousands of supporters filled the streets outside the Barcelona court at the opening of the trial last month.
Many waved the Catalan pro-independence flag, chanting “you are not alone”, “democracy is not a crime” and “independence”.
The current Catalonia government has vowed to hold a new vote in September.
Catalans have often cited Scotland’s 2014 referendum on independence from the UK as an inspiration, and it may be further bolstered by Scottish First Minister Nicola Sturgeon’s announcement that she will seek permission for a second referendum.
Catalan officials say more than 80% of those who voted in its 9 November 2014 poll backed independence.
However, only 2.3 million voters, out of an estimated 5.4 million who were eligible, took part.