He was arrested by Met Police terror officers after earlier being stopped by customs officers.
He is being questioned at a south London police station on suspicion of the the commission, preparation or instigation of acts of terrorism.
A police operation is investigating alleged terrorists flying to Syria.
A police spokesman said the man was arrested at 08:45 GMT at the airport.
"The man was arrested after initially being stopped by immigration officials at 8am," he added.
"Today's arrest forms part of an investigation into travel to Syria in support of alleged terrorist activity."