The elections - Catalonia's third since 2010 - had been expected in 2016.
A symbolic vote on independence - called a "consultation" - was held in Catalonia on 9 November, despite strong opposition from the Madrid government.
In the non-binding poll more than 80% backed independence, Catalan officials said. However, turnout was little more than 40%.
Madrid says any referendum on independence would be unconstitutional.
The timing of the new vote may raise tensions over Catalan separatism again, as Spain is gearing up for a general election, expected in November.
Mr Mas's pro-independence CiU coalition has failed to agree on a joint candidate list with the second-biggest party, the similarly separatist ERC.
But they say that before the vote they will present a common roadmap towards independence from Spain. The region already has a high degree of autonomy.
