The Independent Election Commission (IEC) announcement comes a day after Mr Karzai’s sole challenger, Abdullah Abdullah, pulled out of the race.
Dr Abdullah, who had demanded key poll officials quit, said he did not think it would be a free and fair vote.
The first round of the vote, in August, was marred by mass electoral fraud.
“We declare Hamid Karzai, which [sic] got the majority of votes in the first round and [since] he is the only candidate for the second round… be declared as elected president of Afghanistan,” said IEC spokesman Azizullah Lodin at Monday’s news conference in Kabul.
A Karzai government may mean President Obama changes his position regarding the US efforts in Afghanistan as a “war of necessity”.