Prosecutors had originally asked Brian Thomas be found not guilty by reason of insanity in the 2008 death of his wife, Christine, and be sent to a psychiatric hospital.
On Friday chief prosecutor Iwan Jenkins said "it is now clear that the psychiatrists feel that that would serve no useful purpose and the risk of reoccurrence is very, very small."
The judge ordered the jury return a verdict of not guilty. Thomas, of south Wales, had accepted he killed his wife of 40 years.
The jury was told that Thomas, 59, had a sleep disorder and was in a state of automatism - where his mind had no control over his body - when he strangled Christine.