"I took the decision when looking at the results of the elections of yesterday," Sanchez told the nation in a televised address, Reuters reported.
Sanchez's Spanish Socialist Workers' Party (PSOE) and its junior ally Podemos lost ground during the regional elections held on Sunday, while rival conservative People's Party and far-right party Vox outperformed.
"Although yesterday's elections had a local and regional scope, the meaning of the vote conveys a message that goes beyond that. That is why, as both prime minister and PSOE's secretary-general, I personally assume the results".