Vox is now in power with the conservative Popular Party in the central Castilla y León region just north of Madrid.
Socialist Prime Minister Pedro Sánchez said the coalition was “very bad news”.
Vox is seeking to repeal a law on gender-based violence, opposes gay marriage and wants to centralise Spain by eliminating the 17 powerful regional governments.
The government will pay “close attention” to the policies of the new regional administration, especially with respect to the “rights and freedoms of Spaniards”, government spokeswoman Isabel Rodríguez said Tuesday.
This is the first time a far-right party is sharing power in Spain since the return of democracy after the death of dictator Francisco Franco in 1975.
The move comes ahead of a regional vote expected in Andalusia, Spain’s most populous region, and national elections due at the end of 2023 in which polls suggest Vox is poised to make further gains.
Vox won 13 seats in a snap regional election in Castilla y León in February, up from just one.
The Popular Party came first but fell short of an absolute majority in the 81-seat assembly and joined forces with Vox to remain in power in the rural region north of Madrid.
As he was sworn in for another term as regional president, Alfonso Fernández Mañueco vowed to govern “for all” and said he was “very proud” of the agreement with Vox.
Founded in 2014, Vox started as a marginal force before causing a major upset in late 2018 when it entered the regional parliament of the southern Andalusia region.
The Popular Party tops opinion polls since it appointed a moderate new leader, Alberto Núñez Feijóo on April 2nd but it would need the support of Vox to govern.