“We will do everything to ensure that those who have perpetrated these war crimes do not go unpunished, and therefore appear before the courts… to deal with these alleged cases of (crimes against) humanity, war crimes and why not say it too, genocide,” he said.
“Putin’s unjustified aggression has brought war back to the gates of the European Union”, he told an economic forum in Madrid.
Sánchez is one of the first European Union leaders to label Russia’s actions in Ukraine a “genocide”.
Polish Prime Minister Mateusz Morawiecki on Monday also called for an international investigation into what he termed a “genocide” carried out by Russian troops in Ukraine.
Ukraine and Western leaders have erupted in outrage over the discovery of mass graves and hundreds of dead people in Bucha, a small town northwest of Kyiv.
Ukrainian President Volodymyr Zelensky directly blamed Moscow for the “killings” of civilians.
According to Ukrainian officials, at least 340 civilians killed by Russian troops have been buried in Bucha so far.
“This is genocide. The elimination of the whole nation and the people,” Zelensky told the CBS programme Face the Nation, according to a transcript provided by the network on Sunday.
Russia denied the accusations and said Kyiv had staged footage of the corpses.
Spain’s public prosecutor last month opened an inquiry into “serious violations of international humanitarian law” by Russian troops in Ukraine.