Wang Ching-hsi, an 83-year-old former engineer, is suspected of giving his wife sleeping pills early Sunday and then killing her.
"Why I did this? It's all on my blog," Wang told journalists while being taken away by Taipei police, according to footage shown on local television.
A blog entry dated December 5 which appeared to be Wang's remained available Monday and detailed in candid terms how Wang's 80-year-old wife, Sun Yuan-ping, had lost the will to live.
The blog entry said that she was suffering from Parkinson's disease and recently had broken her left leg. "She is miserable," the blog said.
It referred to an agreement the couple had arrived at a decade ago "to die in peace", and quoted Wang as telling his wife that "when the time comes, I will kill you."
After allegedly killing his wife, Wang called the police to turn himself in and waited in his apartment for the officers to arrive, United Daily News said.
Euthanasia and the right to die has been a hotly debated topic in Taiwan in recent years.
In June, the cabinet passed draft regulations allowing terminally ill patients or their relatives to give up medical efforts that prolong suffering without preventing eventual death.