The stash was discovered at 12:50 p.m. Thursday after agents stopped a 43-year-old man at the Interstate 5 checkpoint, authorities said.
The suspect was hauling a 53-foot-long trailer and after being questioned was directed to an inspection area. A K-9 unit alerted agents to the trailer, which was found to contain 218 boxes filled with marijuana.
The marijuana weighed 18,458 pounds -- about nine tons -- with an estimated street value of $14,766,448, officials said.
"This seizure by our agents helps weaken the transnational criminal organizations who attempt to use our roadways to conduct their illegal activities," San Diego Sector Chief Patrol Agent Paul Beeson said.
The driver is a U.S. citizen whose name was not released. He and the marijuana were turned over to the Drug Enforcement Administration.