The promise came during a meeting on Tuesday between George Bush, the US president, and Fuad Siniora, the Lebanese prime minister, who is visiting the US.
After meeting Siniora in the White House, Bush said they had agreed that al-Hariri's death should be investigated fully, "and we'll work with the international community to see that justice is done".
The former prime minister's assassination triggered demonstrations on the streets of Lebanon that resulted in the final withdrawal a year ago of Syrian troops that had provided the muscle behind Syrian influence in its smaller neighbour for 29 years.
"We took great joy in seeing the Cedar Revolution," Bush said, referring to the uprising.
"We understand that the hundreds of thousands of people who took to the street to express their desire to be free required courage, and we support the desire of the people to have a government responsive to their needs and a government that is free, truly free."
Siniora earlier met Condoleezza Rice, the US secretary of state.
During a three-day visit, he plans meetings with several members of the cabinet and Congress, officials of international lending institutions and Arab diplomats.
The US supports Siniora's goal of a fully sovereign, democratic Lebanon, a point Rice made during a brief visit to Beirut two months ago.
The US wants to end Syrian influence in neighbouring Lebanon, and Lebanese officials said Siniora shared that view.
Bush praised last year's street
demonstrations in Lebanon
Siniora said on Tuesday that he also intended to highlight the importance of US assistance in "backing Lebanon's independence and sovereignty and enabling it to recover its (occupied) Lebanese territories", a reference to the disputed Shebaa Farms area, where the borders of Lebanon, Syria and Israel meet.
That is an area where Hezbollah continues to fight Israel.
After his talks in Washington, Siniora will travel to New York to meet Kofi Annan, the UN secretary-general.