Police in the northern city of Oviedo detained a 51-year-old man late on Monday on suspicion of killing his wife.
A neighbour found the 65-year-old woman's body as well as letter from her husband saying he had killed her, local police said.
A day earlier, police arrested a man in the eastern town of Lliria after he shot dead his ex-wife and former mother-in-law in public. The man had been subjected to a restraining order, the mayor said.
And on Saturday night a 51-year-old man killed his wife before committing suicide in the southwestern town of Baena, police said.
The four deaths bring to 45 the number of women who have died at the hands of their partners or ex-partners so far this year.
The violence came after tens of thousands of people from across Spain marched through Madrid to denounce violence against women.
Protesters answered a call by more than 400 feminist groups to flood downtown Madrid, carrying banners that read "Stop 'machista' violence" and shouting "We aren't all here, the dead are missing!"
In a statement on Twitter, Health Minister Alfonso Alonso slammed the latest violence, saying it had been a "black weekend" for Spain.
And Carmen Benito, head of Women Against Mistreatment, said she was "outraged" by the deaths.
"Measures are needed so this does not happen anymore," she told AFP. One in every eight women - or 12.5 percent - aged 16 and up who live in Spain have experienced violence at the hands of their partners or ex-partners, a 2014 health ministry report found.
Spain is seen as a pioneer in Europe of legislation to combat violence against women, a national rallying point.
A 2004 law introduced an emergency phone number that does not appear on the telephone bill, as well as free legal assistance and accommodation for abused women.
The number of women killed by their partners or ex-partners dropped from 71 in 2003, the year before the law was adopted, to 54 last year.