The 20-year-old American was attacked shortly after midnight on Sunday while the tram was at a stop. Police have said that he intervened when two men harassed a woman on board the tram and was then attacked himself before the men fled. He sustained a deep wound to the face.
The U.S. Embassy in Berlin condemned the "brutal attack" in a post Monday on social network X. "We urge German authorities to swiftly bring the perpetrators to justice and punish them to the fullest extent permitted by law," it said.
A 21-year-old Syrian was detained on Sunday but initially let go because prosecutors didn't at that point see grounds to keep him in custody.
Prosecutors and police said in a statement that the man was arrested on Tuesday morning after new information pointed to him having been involved in the attack. He is now in investigative custody.