A jury has found Shane Claude Roberts not guilty of murdering Rotorua 10-month-old Karlos Stephens but guilty of manslaughter.
The jury returned the 11-1 verdict after 10 hours of deliberation.
He has been remanded in custody for sentencing on February 12.
The trial started last Monday before Justice Sarah Katz and heard by a jury of six women and six men.
Roberts, 61, had pleaded not guilty to murdering baby Karlos Stephens sometime between November 29 and 30, 2014.
The Crown argued the baby suffered a "forceful and likely violent" assault at the hands of Roberts that caused fatal head trauma, but the defence said the baby's mother Pamela Stephens had likely caused the fatal injuries.
Stephens had entrusted the care of Karlos and his twin brother to Roberts in early 2014 after only knowing him a matter of weeks as she had been struggling to look after them.
Roberts, his ex-wife and his two twin daughters all shared the care of the babies.
On the weekend of November 29, Roberts' ex-wife and daughters went to Christchurch and Roberts invited Stephens to stay at his Alison St home to see the boys.
The Crown said Roberts left the boys with Stephens for a short period of time before taking them to his ex-wife's home on Homedale St.
Baby Karlos was injured at this address, where "trails of vomit" were found all over the house.
However, the defence said Roberts had left the boys with Stephens while he was out with friends and picked up an unwell and potentially injured Karlos from her at 3am and took him to Homedale St.
Undisputedly, he arrived at Alison St at 7.30am telling Stephens that "Karlos was not breathing" and the pair drove to the hospital where the 10-month-old died a short time later.
A medical examination found that the baby had suffered from "significant" head trauma that could not have been "accidental", the Crown said.
Crown prosecutor Amanda Gordon said the case was a "tragedy" - for Stephens who was "so desperate" she gave her children to a virtual stranger, but more so for Karlos, who died from a forceful and likely violent assault at the hands of a perpetrator who failed to do anything about it.
Gordon said Karlos had died as a result of trauma that caused bleeding on the surface of his brain and behind his eyes. The force required to hurt Karlos like this was similar to falling from a height or being in a car crash.
There was no evidence Karlos had suffered an accidental injury and it was clear he had been a victim of abusive trauma, she said.
She said Roberts had created a "trail of lies" to cover up what had happened.
Roberts' lawyer Simon Lance spoke about Stephens' previous methamphetamine use, her mental health issues and how she was "prepared to give up her twin boys to a man she barely knew", three weeks after meeting him.
He said she was suffering "significant mental illness and stress" at the time so the jury needed to be cautious when relying on her evidence.
He asked the jury to remember previous defence evidence of Stephens barricading the door and "strangling" another of her sons, he said.
No scene photographs or forensic evidence linked Roberts to Karlos' death and all allegations of lying could have been Roberts trying to protect Stephens, he suggested.
He said the jurors had needed to put their emotions to one side and "focus only on the evidence".