"The Omnium family is getting bigger and bigger, and from today, we have the honour to count on the support of actor Viggo Mortensen as a new member," the group said on Twitter in a post re-tweeted by an account of the actor.
The 59-year-old, who grew up in Argentina and currently lives in Madrid, is in a relationship with Catalan actress Ariadna Gil.
Last year, he came out in favour of an independence referendum in Catalonia, which took place on October 1 despite a court ban.
Founded in 1961 to promote the Catalan language and culture that were then being severely repressed by dictator Francisco Franco, Omnium has in the past years become a major actor in the region's secession movement, organising huge protests along with the ANC, another pro-independence group.
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Its current president, Jordi Cuixart, has been in jail since mid-October, accused of rebellion for his role in the region's failed secession bid.