Joining the court is part of a broader effort by the Palestinians to put international pressure on Israel and comes at a time when the chances of resuming negotiations on Palestinian statehood are seen as slim following Israeli Prime Minister Benjamin Netanyahu's recent election victory and tough campaign rhetoric.
Palestinians signed the court's founding treaty in January and Palestinian membership came into force Wednesday. International justice activists hailed the occasion as an opportunity to bring accountability to years of conflict between Palestinians and Israel.
Israel is not a member of the ICC, but the country's military and civilian leaders could now face charges if they are believed to have committed crimes on Palestinian territory. Israel had no immediate comment Wednesday.
The court's chief prosecutor, Fatou Bensouda, opened a preliminary investigation in mid-January after the Palestinians formally accepted the court's jurisdiction dating back to just before last year's Gaza conflict.
Bensouda was not at the behind-closed-doors welcoming ceremony attended by Palestinian Foreign Minister Riad Malki, as she was out of the country.
Human Rights Watch welcomed the Palestinian Authority as the court's 123rd member and stressed it is now up to Bensouda to weigh whether there is strong enough evidence to merit a full-scale investigation. It is not clear how long her preliminary analysis could take.
"The ICC prosecutor examines allegations of serious crimes no matter the perpetrator, and makes her own determinations about how to proceed based on the evidence" said Balkees Jarrah, international justice counsel at Human Rights Watch. "Any decision whether to pursue an investigation and against whom is not in the hands of the Palestinians or the Israelis."
Prosecution spokeswoman Florence Olara said there are "no timelines" for how long a preliminary examination can take. Some have taken months; others are continuing after years.
The review will likely focus initially on last year's Gaza conflict. The Palestinians suffered heavy civilian casualties in the war, prompting allegations by some rights groups that Israel committed war crimes. Leaders of Hamas, which rules Gaza, could also face charges because the militant group fired rockets indiscriminately at Israeli civilian areas.
Prosecutors could also look at the issue of Israeli settlement building, considered illegal by much of the world. Since 1967, Israel has moved more than 550,000 of its civilians to occupied lands. However, prosecutors in the Hague do not have jurisdiction over events that happened before the Palestinians joined the court.
William R. Pace, convenor of the Coalition for the ICC, which supports and promotes the court's work, said Palestinian membership "gives hope to victims in both Palestine and Israel that they might see justice done and the conflict brought to a peaceful end."