The man, whose identity has not been revealed, was arrested at Bulgaria's main Kapitan Andreevo border checkpoint on Tuesday and charged for "terrorism and arms smuggling," prosecutors said in a statement.
Two automatic rifles, a smoothbore hunting rifle, a SIG Sauer pistol and ammunition for the weapons were found in his car as well as 24 knives and a map of his route showing the Syrian city of Idlib as his final destination, the statement added.
The arrest followed an alert put out by the Swiss authorities after the man's father had reported him and his vehicle missing, together with the arms and ammunition.
The man told prosecutors that he wanted to help civilians in Syria.
A court will decide on Friday whether to extend his detention beyond the initial 72 hours and place him under formal arrest.
Bulgaria is a major crossing point on the route for Europeans heading to join the jihadist Islamic State organisation in Syria and Iraq but border arrests have dwindled since peaking in 2015.
A major military operation is expected in the coming weeks against the opposition-held province of Idlib, with aid agencies warning of a possible humanitarian catastrophe as a result.