The accident took place at about 2am, police said, when a Polish tour coach crashed into a bus from Ukraine. Both vehicles were heading in a westward direction, and they collided near the Dresden-Neustadt city district.
The Polish coach skidded, crashed through the median and collided with a Polish minibus on the opposite side of the highway.
According to police, seven of the nine killed were travelling in the minibus, which was on its way home. Two were in the larger Polish tour coach.
The more than 40 wounded were taken to three Dresden hospitals, as well as additional hospitals in surrounding areas, a spokesman for the city said.
"My thoughts are now with the families of the victims, and with the seriously injured," Dresden Mayor Helma Orosz told dpa, and thanked emergency services for their rapid response.
Highway A4 was expected to be closed around the area of the accident until the afternoon. The highway leads from Eisenach in Thuringia to Gorlitz, on the Polish-German border.
Polish bus operator Sinbad set up an information line for the victims' families. According to the passenger list, there were 65 people on board as well as two drivers and a tour leader.
The Setra 431D-model bus had completed a routine inspection on July 8, according to the company website. Both operators were experienced employees and the bus was on a regularly scheduled route, it added.
Two replacement buses were sent to the crash site, the company said, so that other passengers could decide whether to continue the trip or return to Poland.