Beirut-born Alamuddin has been a legal adviser to the UN in a number of its previous investigations.
The London-based barrister, 36, will join Doudou Diene of Senegal and William Schabas of Canada to serve as members on the commission, the UN said.
At least 1,948 Palestinians, the majority of them civilians, have been killed in the conflict, with 67 Israelis.
Yesterday, hopes for an end to the bloodshed were boosted after Israel and Hamas agreed to a fresh ceasefire. The apparent breakthrough followed renewed violence since the last temporary deal lapsed.
Meanwhile, a Turkish aid group said it would again send ships to challenge the Israeli blockade of Gaza, four years after Israeli commandos stormed its flotilla bound for the Palestinian territory and killed 10 Turks.
The plan looked set to throw a fresh obstacle in the way of efforts to rebuild shattered diplomatic ties between Turkey and Israel, just as Ankara launches an “air corridor” carrying wounded Palestinians to Turkey and aid to Gaza.
Three Palestinian women and a male youth were flown from Tel Aviv to Ankara overnight for medical treatment after Turkey held talks on the matter with Israel, the first step of Ankara’s bid to evacuate possibly thousands from the Gaza Strip.
But any goodwill generated by the move could be jeopardised by the Humanitarian Relief Foundation announcement that a coalition of pro-Palestinian activists from 12 countries had decided to launch a convoy “in the shadow of the latest Israeli aggression on Gaza”.
Nine Turks died in May 2010 after Israeli soldiers raided their vessel, the Mavi Marmara, leading a flotilla to break Israel’s seven-year blockade of Gaza.
A 10th Turkish activist died in May from wounds suffered in the attack.