Alexander Blackman, who had his sentence reduced for shooting dead an injured Taliban fighter, has been released from prison.
The former Royal Marine, who spent more than three years in prison, left Erlestoke Prison, near Devizes, Wiltshire, is the early hours of the morning.
Blackman shot the insurgent, who had been seriously injured in an attack by an Apache helicopter, in the chest at close range with a 9mm pistol in Afghanistan in 2011.
He was originally convicted of murder and sentenced to life imprisonment with a minimum term of 10 years, but this was quashed last month by the Court Martial Appeal Court and replaced with diminished responsibility manslaughter.
The judges ruled that Blackman was suffering from an "abnormality of mental functioning" at the time of the shooting, when he was serving in Afghanistan with Plymouth-based 42 Commando.
The Court Martial Appeal Court found the killing was not a "cold-blooded execution" as the court martial concluded but the result of mental illness, an "adjustment disorder".
The judges described Blackman as "an exemplary soldier" before his deployment to Afghanistan in March 2011 but said he had suffered from "exceptional stressors" during that tour.
They found his ability to "form a rational judgment" was "substantially impaired".
Blackman's sentence was reduced to seven years which meant he was likely to be freed within weeks.
The decision was described by his wife Claire Blackman as "the moment that we have all been fighting hard for".
During the original trial in 2013, Blackman was known only as Marine A.
His identity was made public after his conviction.