Stephen Cahoon, of Harvey Street in the city, had denied murdering 30-year-old mother-of-four Jean Teresa Quigley.
But a jury at Dublin's Central Criminal Court found Cahoon, 43, guilty of the murder at Cornshell Fields in Derry on 26 July 2008 by unanimous verdict.
He had previously been convicted of the murder in 2012, but that was quashed earlier this year.
Cahoon had admitted strangling his Ms Quigley, who was 10 weeks pregnant with his child at the time of her death, but had denied it was murder.
The jury heard that Ms Quigley's naked and bruised body was discovered at her home by her mother.
Cahoon had told the court he handcuffed Ms Quigley to her bed before having sex.
He said he saw "red" when Ms Quigley told him the baby was not his, and he "grabbed and pushed" her and put his hand on her throat.
But a prosecutor told the court there was evidence of "violence from beginning to end" at the scene.
He said there was evidence that the victim's door had been broken and there were defensive injuries and bruising to her arms, scalp and head.
Those injuries were "utterly inconsistent" with Cahoon's version of event, the court heard.
The prosecution lawyer said Cahoon had made a "conscious decision to leave a woman who was unwell and needed assistance and lock her in the house".
"There was no loss of self-control that was temporary and overwhelming to give rise to a defence of provocation," he said.
The trial was Cahoon's third over the murder and he has served almost 10 years in prison.
A hung jury failed to reach a verdict in his first trial in 2009.
In April 2012, another jury found him guilty of murder and he was sentenced to life in prison.
But in March of this year, Dublin's Court of Appeal quashed his conviction due to an error in the judge's instructions to the jury.
A Londonderry man has been found guilty for the second time of murdering his pregnant ex-girlfriend seven years ago.