Tonga will deploy 275 soldiers to Afghanistan over the next two years at the request of the UK Government.
Prime Minister Feleti Sevele put the motion before the nation's Parliament, which voted 22-0 in favour.
An initial contingent of 55 Marines will be deployed in November, the first of four six-monthly rotations of 55 troops who will help guard Britain's Camp Bastion in Helmand province.
The troops will be under British command and Britain will meet the cost of deployment, Mr Sevele said.
Mr Sevele said the deployment would help to ease unemployment in the nation of 104,000 people, as well as fostering closer ties between Tonga and Britain.
Camp Bastion holds a British garrison of around 12,000 troops.
"This is an invitation from the British Army who saw our soldiers work in Iraq and then-Prime Minister Gordon Brown wrote to our prime minister and asked for support," Brigadier Tauaika 'Uta'atu told the Matangi Tonga news website. "This is something we think is an honour to be a part of."
Brig 'Uta'atu said the British government would pay four million US dollars to cover Tongan costs the first year, including uniforms, ammunition, accommodation, travel expenses and a daily stipend for each soldier in Afghanistan.
The soldiers will receive six weeks training in Britain before deployment, likely starting in late September or October.
Tongan officers chose the location for the deployment from among four sites during a visit to Afghanistan in May, the brigadier said.