Sen McCain has repeatedly called for the US to provide military aid to members of the Syrian insurgency.
He becomes the highest ranking US official to travel to Syria, though McCain spokesman Brian Rogers did not give further details about the visit.
News of the trip came as US Secretary of State John Kerry met his Russian counterpart Sergey Lavrov in Paris.
The US currently provides non-lethal aid to opposition groups in Syria, where an estimated 70,000 people have been killed since violence broke out in March 2011.
Rebels call for arms
Sen McCain, the top Republican on the Senate armed services committee, is understood to have entered Syria through Turkey and was on the ground there for several hours.
He travelled with the Syrian Emergency Task Force and met General Salim Idris, chief of staff of the rebel Free Syrian Army, as well as 18 other rebel commanders, the BBC has learned.
Gen Idris called for weapons to continue their fight, as well as a no-fly zone and air strikes on government targets.
These are all steps that Arizona Sen McCain has previously urged the Obama administration to take.
Gen Idris also urged airstrikes on the forces of Hezbollah, the Lebanese militant group whose forces have been fighting in Syria on behalf of President Bashar al-Assad.
Sen McCain -  who was the Republican presidential candidate in 2008 - has repeatedly urged more forceful American support of Syrian rebels, calling for US cruise missiles to target Syrian government forces.
After unverified reports emerged last month that Syrian government forces had used chemical weapons on rebels, the hawkish 76-year-old senator renewed his calls for the establishment of a no-fly zone.
He has also repeatedly urged that the insurgents should be armed.
But the Obama administration has demurred, amid concerns that weapons might fall into the hands of al-Qaeda sympathisers.
Earlier this month, American Ambassador to Syria Robert Ford briefly crossed into northern Syria to meet opposition leaders - in his first visit to the country since he left in February 2012 when the US closed it mission there.
US Senator John McCain has visited Syria to meet rebels in the war-torn country, his office has told the BBC.