The trip came as Cameron appointed a minister to oversee the promised resettlement in Britain of 20,000 Syrian refugees from Middle East camps over the next five years.
The British premier met refugees at a camp outside the town of Terbol in the Bekaa valley in eastern Lebanon, an AFP correspondent reported.
Lebanon hosts more than 1.1 million Syrian refugees, many of whom live in informal tented settlements in the arid valley.
"I'm at a refugee camp in Lebanon, hearing some heartbreaking stories," he tweeted. "British aid is doing so much to help."
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He said Britain was already the second largest donor to Syrian refugee camps in the region.
"We will go on doing that including increasing the amount of money we are giving to educate Syrian children here in Lebanon and elsewhere," he said. "I think that's absolutely vital."
Cameron was also due to hold talks on the refugee crisis with his Lebanese counterpart Tammam Salam.
The British premier has been under pressure internationally and domestically to address the refugee crisis, and vowed last week to "help to stabilise countries where the refugees are coming from".
Britain has accepted 216 Syrian refugees over the past year and granted asylum to less than 5,000 since the conflict broke out in 2011 - far fewer than other European countries like France, Germany and Sweden.
In total, more than four million Syrians have fled abroad.
Forty million pounds of it will be allocated to the United Nations and other non-governmental organisations working with refugees in Jordan, Lebanon and Turkey.