A Hamas official told Reuters that details are expected to be released at a noon news conference in Cairo, where unity talks between the rival factions began Tuesday.
The Western-backed mainstream Fatah party lost control of Gaza to Hamas, considered a terrorist group by the West and Israel, in fighting in 2007. But last month Hamas agreed to cede powers in Gaza to President Mahmoud Abbas' Fatah-backed government in a deal mediated by Egypt.
"Fatah and Hamas reached an agreement at dawn today upon a generous Egyptian sponsorship," Haniyeh said in a statement.
Egypt has helped mediate several attempts to reconcile the two movements and form a power-sharing unity government in Gaza and the West Bank. Hamas and Fatah agreed in 2014 to form a national reconciliation government, but despite that deal, Hamas's shadow government continued to rule the Gaza Strip.