The paper says that Mosab Hassan Yousef, 32, son of a Hamas founder, was employed by Israel's Shin Bet security service and provided information which led to "the exposure of a number of terrorist cells, and to the prevention of dozens of suicide bombings and assassination attempts on Israeli figures."
Yousef, who converted to Christianity a decade ago, now lives in the United States. He has written a memoir, "Son of Hamas," which is being released next week.
Yousef's father was Sheikh Hassan Yousef, a Hamas founder and an influential figure in the West Bank.
A Shin Bet officer identified by Haaretz as "Captain Loai," said that Yousef's help was instrumental in preventing several suicide attacks against Israelis.
"So many people owe him their life and don't even know it," he is quoted as saying. "People who did a lot less were awarded the Israel Security Prize. He certainly deserves it."