Renato Rinino, 41, was shot in bed at his home in the northern Italian town of Savona on Sunday night. Police said the killing was a gangland hit.
Shot in the neck, Rinino died later in the hospital. He became a household name when he confessed in 1997 to having burgled the Prince's apartment in St James Palace in central London three years earlier.
The items he stole included cufflinks, sleevelinks, gold pins, stick pins, watches, brooches and silver boxes, the Buckingham Palace said in a statement at the time.
Rinino also claimed he still had letters addressed to Prince Charles from his long-standing friend, Camilla Parker Bowles. He later returned the booty to the heir to the British throne.
Rinino had recently been arrested at a restaurant in Genoa and placed under house arrest for breaking the terms of his parole and was awaiting sentencing for his latest petty crime.