Hassan Nasrallah said Syria's opposition was too weak to bring down Bashar al-Assad's regime militarily.
He was speaking in an address broadcast on Hezbollah's TV station al-Manar.
BBC Arab affairs analyst Sebastian Usher says the speech tacitly confirmed the group has been involved in fighting in neighbouring Syria.
The Syrian opposition has long claimed the Iranian-backed Shia movement has been supplying fighters to help Mr Assad, a key Hezbollah backer.
"A large number [of rebels] were preparing to capture villages inhabited by Lebanese... so it was normal to offer every possible and necessary aid to help the Syrian army,"  Mr Nasrallah was quoted as saying by AFP news agency.
The Hezbollah leader said it had never hidden its martyrs, but that reports that large numbers of its fighters had been killed were lies.
He also warned that if a key Shia shrine south of Damascus - that named after Sayida Zeinab, a granddaughter of Prophet Muhammad - were to be destroyed, it would spark revenge that could get out of control.
"If the shrine is destroyed things will get out of control," he said.
Mr Nasrallah tried to reassure his domestic audience that - above all - Hezbollah wanted to avoid the Syrian war coming to Lebanon, adds our correspondent, but many there may find little to comfort them in this latest show of defiance.
The announcement came hours after 14 people were killed by a powerful explosion in Damascus, and a day after Syrian Prime Minister Wael al-Halqi survived a car bomb attack in the Syrian capital.
Government forces and rebels have been fighting in and around Damascus for months, but neither have gained the upper hand.
More than 70,000 people have been killed since fighting between Syrian forces and rebels erupted in March 2011.
The head of Lebanese militant group Hezbollah has declared that Syria has real friends who will not let it fall to the US, Israel or Islamic radicals.