The unscheduled stop on Friday is the first by Washington's top envoy to Lebanon since Syria's withdrawal of troops from the country under US and international pressure in April.
Rice arrived on a private plane that landed at 1pm in Beirut, making a detour from Israel in her third trip to the Middle East this year.
She is expected to meet pro-Syrian Lebanese President Emile Lahoud, according to presidential spokesman Rafik Shalala.
Rice was also expected to meet Saad al-Hariri, the son of former Prime Minister Rafiq al-Hariri, whose 14 February assassination sparked anti-Damascus protests that led to
Syria's pullout from the country after 29 years.