The lawyers argued in June that the court has no right to exist and no jurisdiction over Lebanese citizens.
But judges said Monday the court is empowered by a U.N. resolution that Lebanon has accepted.
Hariri, once one of Lebanon's most powerful Sunni leaders, was killed along with 22 others by a truck bomb on Feb. 14, 2005. The court has indicted four members of the Shiite Muslim Hezbollah group which, along with its allies, now holds a majority in Lebanon's Cabinet.
Hezbollah denies involvement in Hariri's killing and has refused to extradite the suspects.