Tillerson's visit, part of a tour of the Middle East, comes at a time of tension between Lebanon and neighbouring Israel, notably over the role of Lebanese Shiite movement Hezbollah.
The Iran-backed group, a member of Hariri's government, is an arch-foe of Israel and branded a "terrorist" organisation by the United States.
It is a key ally of Syrian President Bashar al-Assad and has fighters on the ground in the war-torn country supporting the regime.
Hezbollah's influence was at the heart of a political crisis in Lebanon over Hariri's shock resignation announced from Saudi Arabia in November that was later rescinded.
"The United States stands with the Lebanese people for a free and democratic Lebanon," Tillerson, the highest-ranking US official to visit the country in four years, wrote in the visitors' book after talks with President Michel Aoun.
On Wednesday, speaking in Amman, Tillerson conceded that Hezbollah is part of the "political process" in Lebanon, appearing to soften Washington's tone.
"We support a free, democratic Lebanon free of influence of others, and we know that Lebanese Hezbollah is influenced by Iran. This is influence that we think is unhelpful in Lebanon's long-term future," he said.
"We also have to acknowledge the reality that they also are part of the political process in Lebanon."
Later, US Undersecretary of State Steve Goldstein said however that Lebanon "would be better off without Hezbollah's terrorism and malign influence".
After his talks in Beirut Tillerson was due to head to Turkey seeking to ease tensions with the NATO ally over Ankara's ongoing operation against a US-backed Kurdish militia inside Syria.
