“Special forces police were able to arrest the man without resistance at his home,” the Heidelberg police and prosecutor's office said in a statement.
The suspect, a 66-year-old man living in Ulm and now in custody, had not been known to the police who said he has made no statement yet to them about the case.
“Investigators currently believe that it is unlikely that the suspect had sent other suspected letter bombs,” the police said.
On Wednesday, three people were taken to hospital when a letter bomb exploded at the German headquarters of discount supermarket Lidl.
Around 100 people were evacuated from the administrative building at the company's head offices in Neckarsulm, western Germany.
A similar explosion was also reported in nearby Eppelheim at the Wild drinks company, whose products include Capri-Sun brand.
Authorities also confirmed that an explosive had been identified and disarmed at a parcel distribution centre at Munich airport on Wednesday night.
The parcel, which was addressed to the Bavaria-based baby food company Hipp, was the third such attack in a matter of days.