Hague announced his government intends to make a voluntary contribution of $1.6 million to support the tribunal's work.
"The U.K. is committed to supporting security, stability and justice for the people of Lebanon," he said in a statement. "This contribution to the STL is a signal of our steadfast support for its work to end the climate of impunity for political assassinations in Lebanon."
Hezbollah has said the assassination of Hariri was part of an Israeli plot meant to discredit the Shiite movement. The former prime minister was among 22 people killed in a bomb attack Feb. 14, 2005, in Beirut.
Hague said he was sending Minister of State for the Middle East Hugh Robertson to Lebanon later this week to restate London's commitment to Beirut.
Lebanon's national security is threatened by civil war raging in neighboring Syria. Hezbollah has been fighting alongside supporters of President Bashar Assad.