Paolo Gabriele was arrested on May 23 and placed under house arrest last Saturday after 60 days’ detention in a small "safe room" in the Vatican's police station.
"Paolo has written a letter to the pope asking him for forgiveness, particularly for the pain he caused him," said his lawyer Carlo Fusco.
Gabriele, 46, who worked in the papal household, was arrested after police found a wealth of sensitive Vatican documents in the apartment where he lived with his wife and children inside the Vatican.
Mr Fusco said he expected a Vatican magistrate to formally close an investigation and order Gabriele to stand trial on charges of aggravated theft. It carries a sentence of up to six years in jail under the law of the world's smallest city-state.
If convicted, Gabriele is widely expected to ask the pope for a pardon and if he does so, the pope is likely to grant it, Vatican sources say.
If no pardon is granted, he will serve his term in an Italian jail, according to bilateral agreements between the Vatican and Italy.
Mr Fusco said he had not seen the private letter Gabriele wrote to the pope but that Gabriele told him of its contents. The letter was given to a commission of cardinals investigating a scandal that has become known as "VatiLeaks".
The lawyer said the letter asks for "the kind of forgiveness one would seek from a friend, a spiritual, personal forgiveness". Gabriele was likely seek to a judicial pardon after the trial, Mr Fusco said.
At a news conference late on Saturday when Gabriele was placed under house arrest after a final, seven-hour interrogation, Mr Fusco said Gabriele had wanted to help the pope and may have acted to help clean up the Church.
Many commentators have said that Gabriele, who served the pope his meals and rode in the front seat of the popemobile at the pontiff's general audiences, could not have acted alone and was a scapegoat for others.
But Mr Fusco has said Gabriele was not part of any wider plot and had not received any money.
The butler's arrest in May threw the global media spotlight on an institution battling to defend its reputation against allegations of graft.
Some of the documents which found their way to the media alleged corruption in the Vatican's business dealings with Italian companies, involving the payment of inflated prices for work.
Others highlighted rivalries between cardinals and clashes over the management of the Vatican's bank.