An incendiary device was left at a door of the Oldenburg synagogue in the northern German city of the same name on Friday, the police said.
No one was injured in the attack on Friday afternoon and the perpetrators have not yet been identified, a police spokesman said.
Someone discovered the fire immediately and extinguished the flames. One door was damaged. The police are searching for the perpetrators with a large contingent, and state security is also investigating.
"I condemn this attack on a Jewish place of worship in Oldenburg in the strongest possible terms," said the head of the Oldenburg police, Andreas Sagehorn.
"We will take this attack as an opportunity to immediately increase security measures at the Oldenburg synagogue until the circumstances of the offence have been clarified."
The investigators did not initially provide any further details - such as the background to the offence or how they plan to better protect the house of worship.
