The walkout at the Amazon packing and distribution centre in San Fernando de Henares, near Madrid, kicked off on Monday and was scheduled to last through Wednesday.
The strike is part of a movement launched in March to obtain an increase in wages, which haven't risen since 2016 at the largest Amazon logistics centre in Spain. Strikes have also been called in Germany and Poland.
Ana Berceruelo of the CCOO union said about 80 percent of workers at the site joined the walkout on Monday.
Meanwhile the German trade union Verdi said that employees in German and Poland would also be striking to protest against their working conditions, which includes long periods of standing, walking long distances and repetitive tasks.
Verdi did not specify how many people it expected to participate in the strikes.