Hariri was killed in a massive bombing in downtown Beirut in 2005. The attack on his convoy killed more than a dozen other people.
The Special Tribunal for Lebanon confirmed an indictment against Hassan Habib Merhi for his alleged role in the attack. The STL said it sent Beirut an arrest warrant for Merhi in secret in August so forces could search for, arrest and transfer him to custody.
Beirut said Sept. 6 it couldn't locate the suspect.
Tribunal Judge David Baragwanath called on the Lebanese government to "reinvigorate and intensify their efforts to apprehend."
The tribunal last year released the names of suspects -- Salim Jamil Ayyash, Mustafa Amine Badreddine, Hussein Hassan Ainessi and Assad Hassan Sabra -- who are wanted for the bombing attack.
All four men are suspected members of Hezbollah.
The STL said the Lebanese government is obligated under a U.N. Security Council resolution to help with efforts to capture the suspects.
Lebanon is struggling to ensure a state of national security. Syria's civil war is threatening to spill over its borders.
Hariri was a Damascus opponent and Hezbollah this year admitted to fighting alongside pro-government forces in Syria.