Baltimore Police received a report of the vandalism at 6:30 a.m. Wednesday.
The words "Racist Anthem" have been found painted on a monument to Francis Scott Key on the anniversary of the battle that inspired him to write the "The Star-Spangled Banner".
The monument - primarily concrete and marble, but with gold in the base and a gold figure atop - had been tagged with black lettering, but also with what appeared to be splashes of red and black paint.
The vandalism comes weeks after a monument to Christopher Columbus was damaged in northeast Baltimore. That incident occurred a week after the city removed four monuments honoring Confederate-era figures.
Proponents of removing the monuments say they represent racism and white supremacy, while supporters of the monuments defend them as part of the history of the U.S.
Key, who owned slaves, was inspired to write his verses by the Battle of Fort McHenry on September 13-14, 1814 during the War of 1812.