Simon Bramhall, who admitted carrying out the stunts during transplant operations, was fined £10,000 and given a 12-month community order.
Bramhall, 53, used a jet of argon gas – used for sealing blood vessels – to sign the organs.
He was found out when a colleague spotted the “SB” mark in a new operation after one transplant failed. Bramhall claimed the stunt was intended to relieve tension during surgery.
But Birmingham Crown Court heard that one woman felt like a rape victim after hearing what had happened to her.
Named only as Patient A, she said in an impact statement: “It was what I would imagine the feeling is for someone who is a victim of rape.
“I was meant to be undergoing a life-saving operation. “What was Simon Bramhall thinking of? Why did he think that it was appropriate to do this to me?”
Judge Paul Farrer told Bramhall: “This was conduct born of professional arrogance of such magnitude that it strayed into criminal behaviour. What you did was an abuse of power and a betrayal of trust that these patients had invested in you.”
Bramhall, of Tarrington, Herefordshire, admitted two counts of assault.
Bramhall was the subject of a formal warning by the General Medical Council last year and may face further action following the conviction.
Tony Badenoch, prosecuting, said the bizarre incidents took place at Birmingham’s Queen Elizabeth Hospital in 2013.
He said a nurse working on the second operation saw him marking the organ but when she asked what he was doing, he said: “This is what I do.”
She did not report it but after the liver failed and the patient received a second the surgeon in charge, Darius Mirza, discovered the inch and a half markings.
He asked a nurse to take a picture on his phone and showed it to David Rosser, executive medical director for University Hospitals Birmingham.
Mr Rosser ordered an inquiry which found Bramhall had marked another patient’s liver six months previously – an act witnessed by an anaesthetist.
Bramhall told police he had “flicked his wrist for just a few seconds” to leave his signature on the patient.
Mr Badenoch said: “He said in hindsight this was naive and foolhardy and was a misjudged attempt to relieve the tension in the operating theatre.”
Michael Duck, defending, said some of his former patients were in the public gallery to show their support.
“He accepts that it was a stupid action, and a moment of stupidity for which he is profoundly sorry,” he said.