"After eight days, God stayed their hand from the people of Gaza, and they were compelled to submit to the conditions of the resistance," Meshaal said.
"Israel has failed in all its goals," he told reporters in a Cairo hotel.
Meshaal also thanked ceasefire mediator Egypt, as well as Iran, which he said "had a role in arming" his Islamist movement.
"I would like to thank our dear Egypt, aided by the brave elected President Mohamed Morsi... Egypt acted responsibly and understood the demands of the resistance and the Palestinian people," he said.
Meshaal also praised Iran, despite "disagreements on the situation in Syria".
The truce accord, a copy of which was obtained by AFP, calls on Israel to "stop all hostilities... in the land, sea and air including incursions and targeting of individuals".
It also urges the Palestinian factions to end "rocket attacks and all attacks along the border".
Meshaal said his movement had "rejected Israel's conditions" for the ceasefire "except for stopping operations".
And the Hamas leader warned Israel against violating the agreement.
"If you commit, we will commit. If you do not commit, the rifles are in our hands," he said, adding "we will continue to arm ourselves."
He said an attack on a Tel Aviv bus that injured 17 people on Wednesday "was assisted by God at the right moment to send a message".
Islamic Jihad leader Ramadan Shallah, who sat next to Meshaal, said "what happened was an exceptional failure in the history of" Israel.
"We will strive to acquire all we can to defend ourselves," Shallah said.
Earlier, Egypt announced the ceasefire agreement would come into effect at 1900 GMT on the eighth day of Gaza-linked violence that has killed at least 155 Palestinians and five Israelis.