Prosecutors are seeking a 194-year jail term after Joan Vila Dilme, dubbed "the angel of death", was found guilty by a unanimous jury decision at a court hearing in the northeast town of Girona.
Sentencing will take place later.
The 45-year-old defendant, employed at the La Caritat home in Olot, in Spain's Catalonia region, was arrested in October 2010 after the death of an 85-year-old woman in a local hospital.
He later confessed to her murder, saying he had made her drink bleach, along with the killings of two other elderly people, which he said were carried out in order to "end their suffering".
Eight other murders, committed between August 2009 and October 2010, later came to light, with the victims given overdoses of insulin or various fatal pharmaceutical cocktails in a case which has stirred high emotions in the country.
The defence lawyer said his client suffered mental problems but the jury found no extenuating circumstances.