Trump watched on in horror on a FaceTime call as he witnessed part of a prolonged assault on a woman by Matvei Rumiantsev, seeing the attacker's face before the camera was turned on the weeping victim in an act of humiliation.
As Rumiantsev was sentenced to four years in prison on Friday, Justice Joel Bennathan praised Trump for calling 999 - the emergency number in the United Kingdom - to seek urgent help for the woman.
He said the assault victim - who feared she was about to be killed - had managed to call 999 herself but Rumiantsev snatched away the phone and then dragged her back by her hair when she briefly managed to get outside to try to seek help from a neighbour.
"At one stage in the violence there was a call to or from her friend Barron Trump," the judge said.
"He saw you beating her up and you held the phone and filmed her, an angry act to humiliate her.
"Mr Trump properly and responsibly, despite being in the United States, made sure the emergency services here were called, and he told them what he had seen."
In her evidence to the court, the victim said she believed during the violence which lasted about an hour that she "could not escape and … might die".
The transcript of Trump's 999 call to City of London Police shows he pleaded for urgent action, and was accused of being "rude" when he did not want to go into detail about how he knew the woman.
"I'm calling from the US, uh, I just got a call from a girl, you know, she's getting beat up," Trump said.
When asked how he knew the woman, Trump replied: "I don't think these details matter, she's getting beat up but OK fine, also I met her on social media, I don't think that matters."
When he interrupted the operator to repeat "she's getting beat up," the operator told him: "Can you stop being rude and actually answer my questions? If you want to help the person, you'll answer my questions clearly and precisely, thank you. So how do you know her?".
Trump then said: "I met her on social media."
Later in the 999 call, he said: "She's getting really badly beat up and the call was about eight minutes ago, I don't know what could have happened by now ... So, sorry for being rude."
Trump replied to investigating police in May in an email which said "what I saw was very brief indeed but indeed prevalent," the court heard.
"I didn't expect her to pick up due to the difference in the time zone to the fact I am in the USA, the phone was answered but not by her, to my dismay," the email continued.
"The individual who picked up the phone was a shirtless man with darkish hair, although I didn't get a good look, this view lasted maybe one second and I was racing with adrenaline.
"The camera was then flipped to the victim getting hit while crying, stating something in Russian. The guy had hung up. This whole interaction had lasted five to seven seconds."
In his evidence at trial, Rumiantsev - who hails from a wealthy Russian family and was living in Canary Wharf - admitted he had been jealous at the woman's contact with Trump.
He denied charges against him but was convicted by a jury of assault occasioning actual bodily harm and perverting the course of justice.
The judge passed a four-year prison sentence on Friday.
The judge said Rumiantsev may face deportation when free from prison, he is banned for the next seven years from contacting the woman and the court heard he may be conscripted into the Russian army and deployed to Ukraine if the deportation is successful.
