The children, nine and ten years old, had been playing on the ice on Saturday afternoon when they fell through, several meters (yards) from the shore.
Passers-by saw the incident and alerted the emergency services.
But one 28-year-old man recognized the risk and was able to rescue the girls by crawling on his belly toward them and using a dog leash as a lifeline to pull them out of the ice-cold water.
Other people who were at the scene also helped before the emergency service workers arrived. The girls were then warmed up in the ambulance and were taken to hospital as a precaution.
Germany has experienced a wave of particularly low temperatures over the past week.
While many bodies of water have frozen over, authorities warn about stepping on the ice due to the high risk of breaking through and falling into the water.
