Gordon Thomson, 37, said he splashed two basins of water on Russell Findlay after the incident outside his home.
Mr Thomson also told the High Court in Glasgow that one of the accused, William Burns, 56, was at the scene.
Mr Burns and Alexander Porter, 48, deny attacking the paper's investigations editor in Glasgow on 23 December 2015.
Mr Thomson told the court that he heard a child screaming at about 08:30 and "it was more distressed than playful".
He said he looked out of his window and saw his neighbour, Mr Findlay, wrestling with another man on the ground.
The witness said: "I initially thought it was a burglary. I ran down the stairs and by the time I arrived there Russell was asking the gentleman certain questions."
Mr Thomson told the jury that Mr Findlay was sitting astride the man who was on his back and struggling to get free.
He said that lying nearby was a red Royal Mail jacket and delivery bag and a broken set of false teeth.
The court has previously heard Mr Findlay say that his attacker claimed to be a postman who needed a signature for a parcel, before splashing a "corrosive liquid" on him.
In court, Mr Thomson identified the man on the ground struggling with Mr Findlay as Mr Burns.
Mr Thomson said that Mr Findlay told him he was an investigative reporter and someone had sent a hit on him and had thrown a substance in his face.
The witness said he asked if there was anything he could do and was told by Mr Findlay to fetch water to try to wash the acid off his face.
He added: "His face, the right hand side, was starting to blister and his right eye was starting to close over and was very bloodshot."
The neighbour told the court that he splashed two basins of water over Mr Findlay's face before ambulance staff took over.
Mr Thomson was asked if he heard the man he identified as Mr Burns say anything about who had sent him, and replied: "As he was being led away I heard the words 'Jamie boy sends his regards' or words to that effect."
Under cross-examination Mr Thomson was asked if he heard that or had been told that by Mr Findlay and replied: "It's my recollection that I heard it, but possibly."
PC David Ross later told the court that when Mr Burns was searched after being detained he was wearing two pairs of gloves - one black woolly pair and below that a clear pair of vinyl gloves.
Eye specialist Dr Katheravelu Ramash also gave evidence.
Asked what would have happened if the liquid thrown was sulphuric acid and prompt treatment had not followed, Dr Ramash replied: "He could have lost the eyesight in that eye."
Mr Burns and Mr Porter deny assaulting Mr Findlay to the danger of his life by throwing sulphuric acid on his face.
They also also deny shooting and attempting to murder Ross Sherlock near St Helen's Primary in Bishopbriggs on 24 September 2015.
The trial before Judge Sean Murphy QC continues.
A neighbour who came to the aid of a Scottish Sun journalist after he had acid thrown on him has told a court how his face began to blister afterwards.