In a fiery speech, Sheikh Hassan Nasrallah said he would prove the claim at a press conference next week.
The BBC's Jon Leyne says Mr Nasrallah is trying to pre-empt a UN report due to implicate Hezbollah in the killing.
Mr Nasrallah was speaking hours after a firefight on the Lebanon-Israeli border left five people dead.
Referring to the rare exchange of fire, he said his forces were ready to join the fighting.
Mr Hariri and 22 other people were killed in Beirut in a car bomb in 2005.
In his address - broadcast to a huge crowd in Beirut via video-link - the Shia leader said he would prove that Israeli agents had exploited Hezbollah's "political rivalry" with Mr Hariri, a Sunni Muslim. He did not give any further details.
"I accuse the Israeli enemy of the assassination of (former) Prime Minister Rafik Hariri and... I will prove this by unveiling sensitive information at a press conference on Monday," he said.
Our correspondent says it is the first time Mr Nasrallah has accused Israel of being behind the assassination.
Israel has rejected the allegation, saying Hezbollah was to blame.
"Nasrallah is under heavy pressure," said Israeli Intelligence Minister Dan Meridor, "because of the coming allegations that he or his people (Hezbollah) did kill Rafik al-Hariri, the former prime minister of Lebanon...They may try to divert the attention to another incident."
In July the Hezbollah leader said he was aware that a UN-backed tribunal investigating Mr Hariri's murder was likely to indict members of his party.
He alleged that the court was biased and part of a US and Israeli conspiracy.
Mr Nasrallah's speech was arranged to mark the fourth anniversary of the end of the 2006 war between Hezbollah and Israel.
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