Tourists flocked to the landmark Tuesday to take snapshots of the bronze statue, whose face, left arm and lap had been doused with red paint.
A wooden picture frame sat next to the statue, with the word "AV" - Danish for "ouch" - written in green.
Police said they did not know who had defaced the statue, or whether it was connected to overnight clashes between riot police and rock-throwing youths near Copenhagen's hippie district Christiania.
The Little Mermaid was created by Danish sculptor Edvard Eriksen in tribute to Danish storyteller Hans Christian Andersen. Sitting on a rock at the entrance of the Copenhagen harbor since 1913, she draws an estimated 1 million visitors a year, and has become a popular target for vandals.
Four years ago, the statue was blown off her perch by culprits who used explosives.
She has been beheaded and doused in paint several times. On March 3, vandals covered the statue with pink paint, which was quickly removed by a cleanup crew with soap and high-pressure water cleaners.
The Trump administration is looking for a replacement for the American military contingent in the north of Syria. If the United States agrees with Saudi Arabia, the situation in the south of the country will become a lot more intense as Iran and Israel stand on the brink of war
These armchair generals who come on talk shows or give their opinions as to the capabilities of various military weapons systems are doing no more than inflating their own self image and generating circulation for the news agencies
Syrian military men handed over two unexploded cruise missiles to Russia. The missiles were found after the recent US-led missile strike on Syria