Jihad Hamad, 19, was arrested Thursday morning after turning himself in to police in the city of Tripoli, prosecutors said in a statement. The other main suspect, a 21-year-old Lebanese student identified as Youssef Mohamad el Hajdib, was arrested in Germany last weekend.
The two are accused of planting bombs that failed to explode on two regional trains on July 31.
An international arrest warrant was issued Wednesday for Hamad on suspicion of membership in a terrorist organization, attempting to set off an explosion and multiple counts of attempted murder.
Authorities have said others possibly were involved in the plot, and the prosecutors said in the statement that the investigation continues.
Prosecutors said they would attempt to have Hamad, who lived in Cologne, extradited to Germany.