Mr Hemphill appeared at the High Court in Glasgow
John Hemphill spent more than six years in jail after being found guilty of shooting Sally Cannon in the flat they shared in Kelburn Terrace, Port Glasgow.
But earlier this year appeal court judges ruled that he had not had a fair trial and quashed the conviction.
They accepted that his defence team had not adequately prepared his case, and had not called forensic experts who might have cleared him.
The Crown was given leave to bring him to trial again - and on Wednesday a jury found the charge not proven.
The case against Mr Hemphill, of Highholm Street, Port Glasgow, centred on forensic evidence concerning small spatters of blood on his clothing.
The prosecution claimed he had killed Miss Cannon and that it was her blood.
But Mr Hemphill said the blood had come from Miss Cannon's breath as he cradled her in his arms after finding her dying.
Mr Hemphill, who denied the charge, did not give evidence in the second trial.
Trail of blood
However, the jury heard a police interview in which he told how he arrived back at the flat to find the front door open and blood in the hall and on the stairs.
Fearing that there could be someone in the house, he got a machete from behind the living room fireplace.
He then saw a trail of blood and followed it upstairs, where he found Miss Cannon slumped on a landing.
Mr Hemphill said he ran out of the flat, dialled 999 at a phone box and then went by taxi to his brother's house.
When they returned to the flat Miss Cannon was still lying there, and he said that he cradled her in his arms as they waited for an ambulance.
Expert witnesses
The prosecution claimed that the small blood stains on Hemphill's jacket were consistent with "back spatting" from the entry of a bullet.
However, the defence maintained that the blood could have come from Miss Cannon's mouth as she was dying.
In his appeal earlier this year, Mr Hemphill accused his defence team of failing to call their own expert witnesses to give evidence.
Lord Cameron, who heard the appeal, with Lord Milligan and Lord McCluskey, said they had reached the view that Mr Hemphill had suffered a miscarriage of justice because he was denied a fair trial.