According to the daily Die Welt newspaper, the 53-year-old man was set upon by four teenagers Monday evening at Berlin's Ullsteinstraße U-Bahn station while trying to protect his 22-year-old daughter from an attack.
The teens knocked him to the ground then continued to beat and kick him, before his daughter managed to alert police.
The man was taken to the hospital with severe facial and skull injuries as well as brain trauma, Die Welt reported.
After a short pursuit they arrested a 16-year-old and the two 13-year-olds and are still searching for a 15-year-old boy. All are being arrested on suspicion of aggravated assault.
The incident is the latest in a series of U-Bahn attacks which have left Berliners nervous and on-edge about the system's safety.
In February, four youths were arrested after beating a man into a coma at a Berlin metro station. Just a week later, a homeless man suffered a broken nose after he was beaten up at another station.
Transportation officials said they are working to beef up security by increasing patrols and installing more video monitoring systems.