This is on top of the $1.2 billion the Bush administration pledged earlier and makes the United States the biggest donor to Afghanistan.
Karzai told a news conference in Berlin that the United States also has built 300 schools and 378 clinics in Afghanistan this year, said a transcript released by the U.S. State Department in Washington.
"And please take a message from me back to the United States of extreme gratitude," Karzai told Secretary of State Colin Powell at the joint news conference.
The United States announced its contribution at an international donor-conference in Berlin.
International donors have pledged a total of $4.4 billion for the financial year 2004-05 to help rebuild the Afghan economy.
Afghan Finance Minister Ashraf Ghani said the donor nations also agreed to commit $8.2 billion for next three years.
At a similar conference in Tokyo in 2002, the donors pledged $4.5 billion for five years.
Afghans are seeking $27.5 billion over seven years to rebuild their war-ravaged country.