Emergency services in the eastern region of Valencia confirmed the death toll.
Rainstorms on Tuesday caused flooding in a wide swathe of southern and eastern Spain.
Spain can expect the wet weather to continue through Thursday, forecasters say. (AP PHOTO)
Floods of mud-coloured water tumbled vehicles down streets at frightening speeds.
Pieces of wood swirled with household articles.
Police and rescue services used helicopters to lift people from their homes and cars.
Authorities reported several missing people late Tuesday, but the following morning brought the shocking announcement of dozens found dead.
More than 1000 soldiers from Spain's emergency response units were deployed to the devastated areas.
"Yesterday was the worst day of my life," Ricardo Gabaldón, the mayor of Utiel, a town in Valencia, told national broadcaster RTVE on Wednesday.
He said several people were still missing in his town.
"We were trapped like rats. Cars and trash containers were flowing down the streets. The water was rising to three metres," he said.
Spain has experienced similar autumn storms in recent years, but nothing compared with the devastation during the past two days.
Muddy water tumbled vehicles down streets during the downpour in Valencia. (AP PHOTO)
A high-speed train with nearly 300 people on board derailed near Malaga, although rail authorities said no one was hurt.
High-speed train service between Valencia city and Madrid was interrupted, as were several commuter lines.
Valencian regional President Carlos Mazon urged people to stay at home so as not to complicate rescue efforts, with travel by road already difficult due to fallen trees and wrecked vehicles.
Spain's central government set up a crisis committee to help coordinate rescue efforts.
The rain had subsided in Valencia by late Wednesday morning. But more storms were forecast through Thursday, according to Spain's national weather service.
Spain is still recovering from a severe drought earlier this year.
Scientists say increased episodes of extreme weather are likely linked to climate change.
