Residents were left outraged after noticing the vulgar image on the ground in front of the memorial to 10 US airmen.
The memorial has risen to fame recently after it emerged Sheffield pensioner Tony Foulds had been maintaining its upkeepm since 1969.
Sheffield City Council said it was shocked to see the damage done to the memorial, adding that the vandalism has now been removed.
Councillor Mary Lea, Cabinet Member for Culture, Parks and Leisure at Sheffield City Council, said: “We were shocked and disappointed to find that someone has shown such disrespect to the memorial and what it represents.
“We worked quickly to clean the memorial yesterday and the vandalism has been removed.
“I hope this will not happen again and would ask people to report incidents of vandalism on any of our memorials straight away so that we can deal with them swiftly.”
A Facebook page for Tony Foulds and the Mi Amigo memorial issued a statement criticising the ‘disrespect of the youth of today’.
The message read: “Sad news to report today. I have just found out that mindless cowards have painted obscene images on the ground in front of the memorial.
“To say we are gutted that the youth of today can disrespect the memorial to the youth of yesterday who gave up their lives for their freedom is an understatement. Heartbroken.
“We shall be going down today to check everything else out, including the pictures, wreaths, pots and flowers.”
Mr Foulds was an eight-year-old schoolboy when he saw the plane crash on February 22 1944, killing 10 American airmen.
Since the permanent memorial was built in 1969, Tony said that he has spent 260 days of the year maintaining it.
A flypast was arranged over Sheffield for Friday, February 22,with fighter jets and other military aircraft from Britain and the United States going over Endcliffe Park to mark the 75th anniversary of the crash.