The US stopped its contributions, which make up about a fifth of the agency's funding, when Unesco gave the Palestinians membership in 2011.
Israel, which halted its dues at the same time, has also had its Unesco voting rights suspended.
The US and Israel said admitting the Palestinians was a misguided attempt to bypass the Middle East peace process.
Unesco's loss of $80m (Â£50m) a year in US funding has forced it to pare back American-led initiatives, including Holocaust education and a project to restore water facilities in Iraq.
Paris-based Unesco is charged with designating World Heritage sites, promoting education and supporting press freedom, as well as countering extremism.
The American ambassador to Unesco, David Killion, told Reuters news agency: "We intend to continue our engagement with Unesco in every possible way."
The US, however, has said it cannot legally contribute to a UN agency that implies recognition of a Palestinian state.
Israel's ambassador to Unesco, Nimrod Barkan, told the Associated Press news agency that his country supported the US decision, "objecting to the politicisation of Unesco, or any international organisation, with the accession of a non-existing country like Palestine".
The Palestinian ambassador to Unesco, Elias Sanbar, said other countries were beginning to make up for the funding vacuum left by the US.
But he added: "Is this in the interest of the US, to be replaced?"
The US suspension comes amid attempts by Washington to broker peace negotiations between Israel and the Palestinians.
The talks have hit a stumbling block in the last week over the continued Israeli expansion of Jewish settlements in the occupied West Bank and East Jerusalem.
UN cultural agency Unesco has suspended US voting rights after Washington skipped a deadline for paying its dues.