The formal accession was marked with a ceremony at The Hague, in the Netherlands, where the court is based.
The Palestinians signed the ICC's founding Rome Statute in January, when they also accepted its jurisdiction over alleged crimes
committed "in the occupied Palestinian territory, including East Jerusalem, since June 13, 2014."
Later that month, the ICC opened a preliminary examination into the situation in Palestinian territories, paving the way for possible war crimes investigations against Israelis. As members of the court, Palestinians may be subject to counter-charges as well.
The war between Israel and Hamas militants in Gaza last summer left more than 2,000 people dead, the vast majority of them Palestinians.
, which is not an ICC member, has opposed the Palestinians' efforts to join the body.
"The Israeli position -- like the position of the U.S., Canada and others -- is that the Palestinians are not eligible to join the ICC and the court does not have jurisdiction in this instance, first and foremost, because there is no Palestinian State in international law," Israel's Foreign Ministry said in a statement Wednesday.
"The decision of the Palestinians to join the ICC as an attempt to move 'processes' against Israel is political, cynical and hypocritical."
The Palestinian Authority, which is allied to the "murderous" militant group Hamas, "is the last one that can threaten to present a case to the ICC in the Hague," the ministry added, saying that the Palestinians' membership violated the founding principles of the ICC and risked politicizing the body.
But Palestinian Foreign Minister Riad al-Malki, speaking at Wednesday's ceremony, said joining the ICC was a move toward greater justice.
"As Palestine formally becomes a State Party to the Rome Statute today, the world is also a step closer to ending a long era of impunity and injustice," he said, according to an ICC news release. "Indeed, today brings us closer to our shared goals of justice and peace."
Rights group welcomes Palestinian decision
Judge Kuniko Ozaki, a vice president of the ICC, said acceding to the treaty was just the first step for the Palestinians.
"As the Rome Statute today enters into force for the State of Palestine, Palestine acquires all the rights as well as responsibilities that come with being a State Party to the Statute. These are substantive commitments, which cannot be taken lightly," she said.
Rights group Human Rights Watch welcomed the development.
"Governments seeking to penalize Palestine for joining the ICC should immediately end their pressure, and countries that support universal acceptance of the court's treaty should speak out to welcome its membership," said Balkees Jarrah, international justice counsel for the group.
"What's objectionable is the attempts to undermine international justice, not Palestine's decision to join a treaty to which over 100 countries around the world are members."
U.S.: We will oppose actions against Israel at the ICC
In January, when the preliminary ICC examination was opened, Israeli Prime Minister Benjamin Netanyahu
described it as an outrage, saying the court was overstepping its boundaries.
The United States also said it "strongly" disagreed with the court's decision.
"As we have said repeatedly, we do not believe that Palestine is a state and therefore we do not believe that it is eligible to join the ICC," the State Department said in a statement.
It urged the warring sides to resolve their differences through direct negotiations. "We will continue to oppose actions against Israel at the ICC as counterproductive to the cause of peace," it said.
But the ICC begs to differ with the definition of a state for its purposes and refers to the territories as "Palestine."
While a preliminary examination is not a formal investigation, it allows the court to review evidence and determine whether to investigate suspects on both sides. Prosecutor Fatou Bensouda said her office would "conduct its analysis in full independence and impartiality."
The inquiry will include alleged war crimes committed since June.
The International Criminal Court was set up in 2002 to prosecute genocide, crimes against humanity and war crimes.