Abdelbaset Ali Mohmed al-Megrahi wrote "I am an innocent man" in a letter discovered by The Wall Street Journal at intelligence headquarters in Tripoli, Libya, Monday.
The letter, written in blue ink on an ordinary piece of paper, was apparently composed while al-Megrahi was serving a life sentence in Scotland, the Journal reported.
It was found in a steel four-drawer filing cabinet that had been forced open by rebels who entered the office of intelligence chief Abdullah al-Senussi.
Al-Megrahi, who has been reported seriously ill, was released from prison on compassionate grounds two years ago because he reportedly was dying of prostate cancer.
British Prime Minister David Cameron called the decision to release him incorrect.