He told David Hatto, who has several previous convictions, that he was reducing to two-and-a-half years the four-year sentence passed last week.
Southwark Crown Court had been told how the 26-year-old plasterer and an unknown accomplice pounced on Houda Alouami as she and her mother strolled through Knightsbridge, central London, last August. During the attack the girl was lifted bodily from the ground and a gold diamond-encrusted watch, a birthday present, was ripped from her wrist.
A passing fire crew spotted what was going on and managed to catch Hatto as he hid in a doorway. His accomplice escaped and the watch has never been recovered.
Passing sentence on Friday Judge Jeffrey Rucker told Hatto that because of his 11-year criminal record and the circumstances of the case his initial reaction had been to pass a sentence of about 10 years.
But in view of his guilty plea and the mitigation advanced on his behalf he felt four years was appropriate to protect the public.
Yesterday, however, the judge said he felt that sentence was also too high. "I owe you an apology," he first told Hatto, who is from Manor Park, east London.
He said that he had been considerably affected by the account the girl gave to police of her ordeal and although she had been subjected to an unusual physical attack he felt he had overestimated the need to protect the public from Hatto.
The judge said he believed that he had also overestimated the seriousness of the offence. He added: "I also failed to give enough effect to the mitigation before me. Above all I think the light has at last dawned on you that your salvation is your family and that crime leads nowhere but behind bars. I believe you have turned the corner from crime and won't offend again."
Hatto, on licence from a 33-month sentence for violence and other offences when he attacked the girl, responded: "Thank you."
Michael Stephen, vice-chairman of the Conservative home affairs committee, reacted angrily to the judge's statements. He said: "The public clearly need protecting from the judges as well as the criminals. It seems that this is manifestly lenient for a terrifying attack on a young girl and the theft of property worth pounds 12,000."
Chris Sallon QC, a leading barrister, said the judge's decision illustrated why mandatory sentencing would not work.
In a veiled criticism of the reforms unveiled by Michael Howard, Home Secretary, in his Crime Bill, Mr Sallon said: "This illustrates how important it is for judges to be able to sentence in a flexible way and how mandatory sentencing could lead to injustice in the eyes of the judge."