"It is a strong act of confidence in UNESCO and in multilateralism," said its director general Audrey Azoulay when she informed representatives of the body's member states in Paris of Washington's decision to rejoin.
The United States, a founding member of UNESCO, was a major contributor to UNESCO's budget until 2011, when the body admitted Palestine as a member state.
That triggered an end to the contributions under U.S. law.
Then president Donald Trump went further by announcing in 2017 that the United States was withdrawing from UNESCO alongside Israel, accusing the body of bias against the Jewish state.
Its pullout took effect in 2018.
Azoulay, a former French culture minister who has headed UNESCO since 2017, has made it a priority of her mandate to bring the U.S. back.
In a letter to Azoulay seen by AFP, Richard Verma, the U.S. Deputy Secretary of State for Management and Resources, said Washington was "grateful" to Azoulay for progress on "significant issues," including "decreasing focus on politicized debate."
Until the suspension of its contributions in 2011, the U.S. paid about 22 percent of UNESCO's budget, or $75 million.
But the U.S. Congress, then fully controlled by President Joe Biden's Democratic Party, in December paved the way for the United States to restore funding, setting aside $150 million in the budget.
U.S. Secretary of State Antony Blinken in March said the US absence from UNESCO was letting China write rules on artificial intelligence.
"I very much believe we should be back in UNESCO - again, not as a gift to UNESCO, but because things that are happening at UNESCO actually matter," Blinken told a Senate committee when he presented the budget.
"They are working on rules, norms and standards for artificial intelligence. We want to be there," he said.
The U.S. had already withdrawn from UNESCO in 1984 and rejoined the Organization after an almost 20-year absence, in October 2003.
The proposed plan to rejoin in 2023 must now be submitted to the General Conference of UNESCO Member States for final approval.