Two of al-Rantisi's bodyguards were also killed in the attack on Saturday evening.
Medics said al-Rantisi was taken to a Gaza City hospital in a critical condition after the attack. He had reportedly been wounded in the head with shrapnel.
Hundreds of outraged Hamas members and supporters flocked to the hospital after news of the Israeli raid.
The air strike occurred hours after an Israeli border policeman was killed by a Palestinian bomber at the Erez crossing on the Israeli-Gaza border.
No immediate comment was available from the Israeli army on the helicopter strike.
Al-Rantisi, a co-founder of Hamas, has become one of its two main leaders since Israel's killing of Hamas spiritual head Ahmad Yasin in Gaza on 22 March.
Hamas chief (C) was the public
face of the resistance group
Israel tried to kill al-Rantisi, the public face of a Palestinian resistance group that normally stays in the shadows, last June.
On that occasion he and his teenage son were wounded in an Israeli helicopter missile strike on his car, also in Gaza City.
Al-Rantisi had refused to go into hiding like many of his comrades on Israel's wanted list since Hamas launched a human bombing campaign to spearhead a three and a half-year-old Palestinian uprising.
He had long depicted himself as a Hamas politician with no links to the military wing.
But Israel had refused to accept the distinction, accusing him of being a top decision-maker on attacks and of using his media role to incite violence.
With al-Rantisi filling the role of Hamas spokesman, camera crews from around the world have trooped to his modestly furnished living room to hear him issue vows of revenge, often in calm, even tones, for Israel's killing of resistance activists.