A Lebanese judicial source said that the man, identified only by the initials AH, was arrested on Monday based on information provided by two suspects already in Lebanese custody.
The source said the man, from the northern Lebanese town of Akkar, was in his twenties but gave no further details.
Ahmad Fatfat, Lebanon's acting interior minister, said on Friday that Jihad Hamad, a 20-year-old student who turned himself in to authorities a day earlier, had confessed to planting a suitcase bomb and investigations suggested that he might have links with al-Qaeda.
Germany already holds two suspects, including a 21-year-old Lebanese named Youssef Mohamad el-Hajdib.
El-Hajdib was identified on security camera footage that appeared to show him dragging a suitcase into a train in Cologne last month.
Suitcases like those in the footage were found packed with propane gas tanks and crude detonating devices on trains in Dortmund and Koblenz.