An illustrative image show the Palestinian flag and Portuguese flag waving side by side. Photo courtesy of WAFA.
Britain, Canada and France are among the other Western nations planning to give a Palestinian state recognition at the assembly, which comes as Israel's campaign against Hamas in the Gaza Strip grinds on.
Lisbon had already announced in July that it intended to do so, given the "extremely worrying evolution of the conflict", as well as the humanitarian crisis and Israel's repeated threats to annexe Palestinian land.
"The Ministry of Foreign Affairs confirms that Portugal will recognise the State of Palestine... the official declaration of recognition will be made on Sunday, September 21," a statement on the ministry's website said.
Israel has fiercely criticised plans for Palestinian recognition.
Yet the spiralling humanitarian crisis in the coastal strip, where the United Nations has warned of famine in Gaza City, has convinced even some of Israel's longtime allies to recognise a Palestinian state.
Earlier Friday, an adviser to French President Emmanuel Macron said Andorra, Australia, Belgium, Luxembourg, Malta and San Marino also plan to recognise the State of Palestine.
Beginning on Monday, next week's UN General Assembly in New York is set to be devoted to the question of the two-state solution to the long-running Israeli-Palestinian conflict.
Last week, the UN General Assembly overwhelmingly voted to back a declaration outlining “tangible, time-bound and irreversible steps” toward a two-state solution.
Around three-quarters of the 193 UN member states already recognise the state of Palestine, proclaimed in 1988 by the exiled Palestinian leadership.
In 1947, the 181 resolution of the UN General Assembly decided on the partition of Palestine, then under a British mandate, into two independent states -- one Arab and the other Jewish.
Yet, while Israel was established the following year, in the Palestinian Territories, the Arab state never materialised, as the United States and Israel obstructed its creation, with Israel ultimately occupying and confiscating more and more Palestinian land.
The right to establish a state, as enshrined in the UN Charter's principle of self-determination, "cannot be subject to Israel’s veto", and the UN has never conditioned Palestine’s existence on Israeli acceptance.
Short link: