"We're still investigating what happened and making sure we have the whole picture," police spokesperson Oliver Pohl told The Local.
Witnesses says that the 55-year-old man from Kiel yelled "shit Muslims" before launching an unprovoked attack on the 35-year-old woman.
Police say she suffered multiple bruises and a broken nose as a result of the attack.
The Kieler Nachrichten published a photo of the victim, showing her with two black eyes.
"It all happened in seconds," one witness told the newspaper. "It was so brutal - like a boxer delivering a knock out punch."
Hate crimes across Germany rose dramatically in 2015 after German took in a massive influx of refugees. Islamophobia has also become much more widespread, a recent study shows.
State police spokesperson Torge Stelck told The Local that the state had also seen an increase in politically-motivated hate crimes.
From 2014 to 2015, such categorized crimes doubled from 33 to 66 statewide, with "the majority" of those being perpetuated by right-wing extremists.