The group was climbing the Mayon volcano when an explosion happened at about 8 a.m., lasting just over a minute, the Philippines' National Disaster Risk Reduction and Management Council said in a statement.
Rocks “as big as a living room” came raining down, guide Kenneth Jesalva told ABS-CBN TV, according to the Associated Press.
Seismologists described Tuesday's event as a routine, steam-driven explosion that sent a thick cloud billowing more than 1,600 feet into the air.
A total of 27 hikers were on the volcano at the time, according to the disaster management council. In addition to the Germans, at least five others were injured, all Filipinos.
The 8,070-foot volcano in Albay province, about 200 miles southeast of Manila, is known for its near-perfect cone.
A 3.7-mile "permanent danger zone" has been declared around the volcano. But local authorities told the French news agency AFP that they allow people to climb when there are no signs of an eruption.
After Tuesday's incident, provincial Gov. Joey Salceda told ABS-CBN TV that all "human activities" within the danger zone had been suspended.
Tens of thousands of people were evacuated when the volcano erupted in 2009.