Saudi Arabia announced on Sunday that it was giving $3 billion to strengthen the Lebanese army, amid growing sectarian strife in Syria that is spilling into Lebanon.
Lebanon's president, Michel Suleiman, made a surprise announcement on television that the money would be used to buy weapons from France.
Suleiman did not elaborate on the deal but said that it was the largest pledge of money ever towards the Lebanese army.
He said that France's president Francois Hollande would discuss the deal with his Saudi counterpart on Sunday during a visit.
"I am happy to tell the Lebanese people that the Saudi ruler will give a grant of $3 billion to strengthen the army," Suleiman said.
"The Saudi grant will allow the Lebanese army to purchase weapons from France."
The news comes amid growing regional instability and sectarian violence.
The Syrian conflict, which has divided some in Lebanon, has threatened to spill into fragile Lebanon for the last several years.
Deadly bombings and shootings have rocked the country with the blame put on the instability caused by the Syrian civil war.
The latest wave of violence came when a popular anti-Assad, Sunni politician was killed in a car bomb.
Tripoli, Lebanon's second largest city, has been the site of near-constant battles between Sunni and Alawite militias that battle in the city's poorest neighborhoods.
The Lebanese army has been criticized for not being able to keep the peace, mostly due to a lack of resources.