Spain's governing Socialists won the country's third election in four years, but have fallen short of a majority.
PM Pedro Sánchez's party polled 29% and will need the help of either left-wing Podemos and regional parties, or the centre right, to form a government.
For the first time since military rule ended in the 1970s, a far-right party is set to enter parliament.
Vox opposes multiculturalism, unrestricted migration, and what it calls "radical feminism".
Analysts say support for Vox has been boosted by widespread anger at the Catalan independence drive. The party fervently opposes any concessions to the secessionists.
The other big story of the election was the collapse in support for the Popular Party (PP), which governed Spain until it was dumped from power in May 2018 in a no-confidence vote.
In its worst election ever, the PP won just 66 seats, down from 137 in the previous parliament.
Turnout was 75.8%, the biggest for several years and 9% higher than the previous election in 2016.