Khaled Meshaal, the exiled leader of Hamas, is on Palestinian land for the first time in 45 years.
Mr Meshaal, who went into exile in 1967 and now lives in Qatar, is in Gaza to mark the 25th anniversary of the founding of Hamas, which controls the territory.
The BBC reports he had not set foot in the Palestinian territories for decades. In a statement, he said:
"I consider this moment my third birth, and I pray to God that my fourth birth will be the moment when all of Palestine is liberated.
"Gaza is always in my heart."
He said his previous two "births" were the day he survived an assassination attempt by Israeli agents in Jordan in 1997, and his actual birth in 1956.
Mr Meshaal is expected to visit the home of late Hamas founder Sheikh Ahmed Yassin, as well as that of Ahmed Jabari, a military commander killed in an Israeli strike last month.
Egypt's former president Hosni Mubarak never allowed him to use the Rafah border crossing to enter Gaza, and it was seen as too risky in terms of his personal security.
An Israeli official told the BBC that no guarantees for Mr Meshaal's safety in Gaza had been requested and none had been given.
"Hamas is Hamas, it doesn't matter who heads it," a foreign ministry spokesperson said.
Israel, the United States and the European Union consider Hamas a terrorist organisation.