The United States' closest allies were among the first to congratulate Biden.
British Prime Minister Boris Johnson called the win a "historic achievement."
"The U.S. is our most important ally and I look forward to working closely together on our shared priorities, from climate change to trade and security," he tweeted.
Canadian Prime Minister Justin Trudeau said he looks forward to working with Biden and Harris in the future.
"Canada and the United States enjoy an extraordinary relationship -- one that is unique on the world stage. Our shared geography, common interests, deep personal connections and strong economic ties make us close friends, partners and allies. We will further build on this foundation as we continue to keep our people safe and healthy from the impacts of the global COVID-19 pandemic, and work to advance peace and inclusion, economic prosperity, and climate action around the world," he said.
Fiji's prime minister, Frank Bainimarama, said the two leaders need to work together to address global warming and rebuild the economy.
"Congratulations, @Joe Biden," he tweeted. "Together, we have a planet to save from a #ClimateEmergency and a global economy to build back better from #COVID-19. Now, more than ever, we need the USA at the helm of these multilateral efforts (and back in the ParisAgreement - - ASAP!)."
Other world leaders' congratulations poured in as Biden crossed the 270-electoral vote threshold to win the presidency with a win in Pennsylvania.
Irish Prime Minister Micheal Martin called the president-elect "a true friend of this nation."
"I heard a Pan-European sigh of relief, when Biden's victor was called," German member of the European Parliament, Reinhard Butikofer, said.
Harris made history as the first woman of color elected U.S. vice president.
Scottish First Minister Nicola Sturgeon hailed "President-Elect Joe Biden and the history-making Vice President-Elect Kamala D. Harris."
The Times of India projected Biden's win with the headline, "Bye don, it's Biden finally."
The Indian English-language daily newspaper said that H1-B work visas that allow non-immigrants to work in the United States are unlikely to return to their previous numbers, but a Biden administration could be stronger on human rights violations in India.
The newspaper also detailed celebrations in Harris' ancestral village in southern India, the birthplace of her maternal grandfather.
Iranian President Hassan Rouhani has urged the United States to return to its commitment under the nuclear deal negotiated by the Obama-Biden administration, which President Donald Trump withdrew from two years ago.
"For us, the individual and the party are not important; rather, what matters is the policies to be adopted by the U.S. government," Rouhani told the Islamic Republic News Agency.
Tehan's City Council chairman, Mohsen Hashemi Rafsanjani, said Saturday the targeted killing of Qassem Soleimani, the former commander of Iran's elite Quds Force, by a U.S. military drone strike in January near Baghdad helped cause "Trump's heavy defeat," BBC Persian reported.
Biden has promised to rejoin the World Health Organization after Trump formally withdrew, claiming bias toward China.
Biden is a "globalist at heart," wrote Natasha Kassam, a research fellow at Sydney's Lowy Institute political think tank, in an opinion-piece in The Guardian.
Rebecca Lissner, a non-resident scholar at Georgetown University's Center for Security Studies, told Euronews Biden will reaffirm old alliances.
Lissner said that she also expects Biden to modernize alliances so that they are "less like relics of the Cold War and more like the type of international tools that we need to defend against the full spectrum of 21st-century security challenges, many of which are not explicitly military in nature."
Trump has continued to make unsubstantiated allegations of electoral fraud and in Georgia, a recount is all but certain, elections officials said.
In Russia, President Vladimir Putin's supporters cited Trump's false claims of widespread election fraud as proof that democracy is a recipe for disaster.
Experts say a Biden-Harris administration will bring a more calculated approach to foreign relations.
"On North Korea and so many issues, Biden and Harris will bring a coherence and a competency that has been lacking for four years," said John Delury, a professor at Yonsei University's Graduate School of International Studies. "They will take the North Korea issue really seriously, in a more disciplined and organized way. It doesn't mean they'll come to the right conclusion, but the process will be back to a more normal U.S. government process."
The United States' image hit a record low in six countries, including Britain, France, Germany, Canada and Australia, within a few months of the election, a Pew Research Center poll showed, linked in part to bad ratings for its handling of the COVID-19 pandemic.
Trump was less trusted than leaders of Germany, France, Britain, Russia and China to do the right thing regarding world affairs, according to the poll.
Still, a handful of world leaders voiced support for Trump, including those with anti-immigration views, such as Hungary's right-wing nationalist prime minister, Vikto Orban. Other presidents who supported Trump include Brazilian President Jair Bolsonaro, who has also downplayed the COVID-19 pandemic and climate change. Philippine President Rodgrigo Duterte, under whom human rights abuses have worsened amid ongoing war on drugs, had also endorsed Trump.