Speaking hours after Karzai vowed to battle graft as he was sworn in to a second term, the minister said the allies were awaiting concrete action from Kabul.
"Today was an interesting day in Afghanistan, and we will see what will be the result of it," zu Guttenberg told reporters after meeting his US counterpart.
"We need more than just words. We need action also in Afghanistan by the Afghan government," he said outside the Pentagon.
In his inauguration speech, Karzai described corruption as a "dangerous problem" and promised to organize a conference to find ways to counter the scourge.
The German minister said his government remained steadfast in its commitment to the NATO-led mission in Afghanistan.
"I told him (the US defence secretary) that the German commitment regarding Afghanistan is firm," he said.
Germany has 4,500 troops deployed in Afghanistan, the third largest contingent in the international force. Of more than 100,000 foreign troops, about 68,000 are US forces.
He said Germany was keen to hear the outcome of a US strategy review on Afghanistan by President Barack Obama, who is weighing whether to send more American troops.
"We all are eagerly waiting for the president's speech and for the new concept, for the new strategic ideas from our American friend."