Speaking at a press conference in Kabul on November 3, he also pledged to eradicate what he called "the stain" of corruption in Afghanistan.
Karzai's only challenger, Abdullah Abdullah, withdrew from the vote on November 1, citing doubts about the fairness of the process.
On November 2, U.S. President Barack Obama congratulated Karzai on securing another term and urged him to do more to fight corruption.
White House Press Secretary Robert Gibbs said Obama was relieved that a complicated Afghan electoral process had concluded.
"I think by all accounts, this has been a difficult process," he said. "This is the first election run by the Afghans. But I think the president, the [U.S.] Embassy there, and everyone can take heart in the notion that the laws of Afghanistan and the institutions of Afghanistan prevailed."
At the State Department, spokesman Ian Kelly said the United States is "prepared to work with this partner, who was elected according to Afghan laws in an election that was conducted by Afghan institutions."
He added, "We have a big stake in Afghanistan. The international community has a big stake in Afghanistan. We stand ready to support them as they go forward."
EU External Affairs Commissioner Benita Ferrero-Waldner said Abdullah's withdrawal from the presidential campaign and the cancellation of the runoff won't change the EU's plans to increase aid to Afghanistan.
Speaking to journalists in Kabul on November 2, Azizullah Ludin, who heads the Afghan Independent Election Commission (IEC), said "the economic and political fallout of this protracted process" and the prevention of "uncertainty and challenges to the security" were the main reasons behind the commission's decision to call off the runoff.
"In the light of all these arguments and in line with Article 156 of the constitution and Article 49 of the election law," he said, "the election commission is declaring Mr. Hamid Karzai, the leading candidate in the first round of the election and the only candidate in the second round of the election, as the elected president."
The announcement came shortly after visiting UN Secretary-General Ban Ki-moon had called for a decision on the election "as soon as possible." Afterward, Ban said he welcomed the IEC's decison to forego the runoff and declare Karzai the winner.
Earlier in the day, Ban had met separately with Karzai and former Foreign Minister Abdullah Abdullah, who on November 1 withdrew from the second-round runoff because he felt a fair and transparent vote was not possible.
Speaking to journalists in Kabul, Ban expressed hope that the Afghan election commission would uphold the law in deciding whether to go ahead with the vote.
"I am sure that the due process and observance of the law will prevail. And the Afghan Independent Election Commission will apply constitutionally correct procedures," Ban said.
Western diplomats were reportedly not in favor of holding the second-round vote following Abdullah's pullout.
The U.S. Embassy in Kabul and the British government have both welcomed the IEC's decison and congratulated Karzai on his victory.
The runoff was called after a United Nations-backed investigation found that widespread fraud, mainly in favor of Karzai, had been committed during the first-round vote held on August 20.
The UN secretary-general is in Kabul to show solidarity with UN election workers after an attack on a UN guesthouse in the Afghan capital killed five UN international staff and three Afghans last week.