Mr Brown stayed overnight at Kandahar Airfield, the coalition headquarters for the region, and was briefed by senior military staff. He met troops and inspected new equipment purchased in recent months.
The Prime Minister later flew to Camp Bastion, in Helmand province, where the bulk of the UK's troops are based.
At a joint press conference with the Afghan President, Hamid Karzai, Mr Brown said the rising level of British troops was being met by increases in equipment, including helicopters and IED surveillance devices. He added:
"We have ramped up our counter-IED efforts to give the best protection possible to our troops on the ground."
The PM said the plan was to continue to weaken the Taliban and to strengthen the Afghan authorities:
"I think the next few months are obviously critical. We need to show there is support for our forces back in Britain, which I know there is, and a determination to take on the Taliban."
Mr Brown thanked troops for their 'bravery, professionalism and dedication' in what he described as a difficult year. And he paid tribute to Service personnel who would be apart from loved ones over Christmas.
It is the Prime Minister's third visit to Afghanistan this year.