The London-based lawyer will join Doudou Diene of Senegal and William Schabas of Canada to serve as members on the international commission, the UN said.
It said in a statement: "The United Nations Human Rights Council announced today the appointment of three members to its independent Commission of Inquiry to investigate purported violations of international humanitarian and human rights laws in the Occupied Palestinian Territory, including East Jerusalem, and particularly in the Gaza Strip since the conflict began on June 13.
"The commission aims to establish the facts and circumstances of violations and crimes perpetrated and to identify those responsible. It will also make recommendations, in particular on accountability measures, all with a view to avoiding and ending impunity and ensuring that those responsible are held accountable, and on ways to protect civilians against any further assaults."
At least 1,948 Palestinians, the majority of them civilians, have been killed in the conflict, with 67 Israelis, according to UN figures.
The situation has led to calls to investigate Israel for war crimes, with comparisons being drawn to the Holocaust.
The Council decided - by a vote of 29 countries in favour - to launch the inquiry at its emergency meeting on July 23. The commission is to present a written report to the Human Rights Council at its session next March.
Yesterday, hopes for an end to the bloodshed were boosted after Israel and Hamas agreed to a fresh ceasefire.
The apparent breakthrough follows renewed violence since the last temporary deal lapsed.
More than a dozen Palestinians are said to have died in Israeli airstrikes over the weekend, while mortar and rocket fire from Gaza resumed.
But the Israeli military said no rockets have been fired at Israel today and it had not targeted any locations in the densely populated territory since the ceasefire went into effect at midnight local time.
Alamuddin, 36, is a member of Doughty Street Chambers in London, having joined in 2010 to complete her pupillage
The Oxford-educated barrister, who has represented WikiLeaks founder Julian Assange, has been described as a "passionate defender of human rights".
She was previously counsel to the inquiry led by the UN into the use of drones, and was legal advisor to former secretary-general Kofi Annan on the Syria conflict and the head of the commission investigating the assassination of former Lebanese Prime Minister Hariri.
She has also represented clients in cases before the International Criminal Court, International Court of Justice and the European Court of Human Rights.