"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.
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.
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![33 passengers and one crew member died on the dive boat, Conception [Brian van der Brug/Los Angeles Times/AP]](data:image/svg+xml;charset=utf8,%3Csvg%20xmlns%3D'http%3A%2F%2Fwww.w3.org%2F2000%2Fsvg'%20width%3D'300'%20height%3D'172'%20data-ring-placeholder%3D'1'%3E%3C%2Fsvg%3E "33 passengers and one crew member died on the dive boat, Conception [Brian van der Brug/Los Angeles Times/AP]")
