Organisers of the Rock am Ring festival, which is headlined by US rock band Red Hot Chili Peppers, said lightning hit at 12.30 a.m., BBC reported.
Police told German media two people had to be resuscitated.
Central Europe has been hit by severe storms and rain over the past few weeks, leading to 11 deaths in Germany.
The festival also suffered lightning strikes last year, when 33 people were taken to hospital.
The organisers of Rock am Ring, now in its 31st year, said this year's event would continue, and that clear weather was expected later on Saturday.
Some 45,000 people are expected to attend over the course of the weekend.