The decision by the 57-member grouping came at an emergency meeting in Jeddah, Saudi Arabia, three days after the Arab League ratified the plan at a summit in Cairo.
The Egyptian-crafted alternative to Trump’s widely condemned takeover proposes to rebuild the Gaza Strip under the future administration of the Palestinian Authority.
“The emergency ministerial meeting of the Organisation of Islamic Cooperation adopted the Egyptian plan, which has now become an Arab-Islamic plan,” Egyptian Foreign Minister Badr Abdelatty said, in comments echoed by his Sudanese counterpart.
“It is certainly a very positive thing,” Abdelatty said.
Trump triggered global outrage by suggesting the US “take over” Gaza and turn it into the “Riviera of the Middle East”, while forcing its Palestinian inhabitants to relocate to Egypt or Jordan.
At Tuesday’s summit in Cairo, Arab leaders also announced a trust fund to pay for Gaza’s reconstruction and urged the international community to back it.
“The next step is for the plan to become an international plan through adoption by the European Union and international parties such as Japan, Russia, China and others,” Abdelatty said.
“This is what we will seek and we have contact with all parties, including the American party.”
However, the counter-proposal does not outline a role for Hamas, which controls Gaza, and was rejected by both the United States and Israel.
The plan “does not meet the expectations” of Washington, State Department spokeswoman Tammy Bruce told reporters on Thursday.
Trump’s Middle East envoy Steve Witkoff gave a more positive reaction, calling it a “good-faith first step from the Egyptians”.
Rabha Seif Allam, of the Al-Ahram Center for Political and Strategic Studies in Cairo, said Egypt was seeking “broad support” for its proposal.
“This is an attempt to build a broad coalition that refuses the displacement” of Palestinians from Gaza, she said.
Trump’s plan has already united Arab countries in opposition, with Saudi Arabia also hosting Arab leaders two weeks ago to discuss alternatives.
The OIC also readmitted Syria, which was suspended in 2012 early in the civil war under Bashar al-Assad, following the long-time ruler’s toppling in December.
“This decision represents an important step towards Syria’s return to the regional and international communities as a free and just state,” a Syrian foreign ministry statement said.