The man, who was later released by a judge pending further inquiries according to a judicial source, was arrested on Wednesday in Madrid a police spokesman told AFP.
“He said his wife was terminally ill and that he had given her a substance to provoke her death so that she no longer suffered,” the spokesman added.
During an interview with private television La Sexta broadcast last year, the man, Angel Fernandez, said his wife, who was in her early 60s, had battled multiple sclerosis, for 30 years, was in pain and extremely tired, and wanted to die “with dignity”.
“The ideal would be if a professional was authorised (to help her die) but as it's impossible, if she asks, I should do it,” he added.
The news of the man's arrest reopened the debate over euthanasia as Spain gears up for a snap general election on April 28th, with the case dominating daytime television talk shows and sparking a flurry of opinions on social media.
READ MORE: Spain takes steps to legalise euthanasia
Socialist Prime Minister Pedro Sanchez has promised to make Spain the fourth country in Europe to legalise euthanasia after Belgium, Luxembourg and the Netherlands if he wins a majority in parliament — a move fiercely opposed by the main opposition conservative Popular Party (PP).
“We want people not to suffer beyond what they freely decide,” Deputy Prime Minister Carmen Calvo wrote on Twitter on Thursday.
The Socialists in June presented a draft law on legalising euthanasia which was backed by far-left party Podemos but the but the PP and centre-right Ciudadanos blocked it in a parliamentary committee in October.
Ciudadanos secretary general Jose Manuel Villegas said his party was in favour of the law in general but would prefer that another law which it proposed on palliative care was approved first.
“What this case highlights is that we need this regulation,” he told a news conference in a reference to the man's arrest.
The PP declined to comment on the case, arguing that party leader Pablo Casado had yet to present the election programme.
The party said last year there was no need for a law allowing euthanasia.
Polls show a majority of Spaniards back euthanasia for terminally ill patients are mentally unimpaired. Multiple sclerosis is a rare, disabling autoimmune disease that damages the central nervous system. It can lead to fatigue, pain, vision loss and impaired coordination and motor skills.