The banner fluttering across the front of the Trump International Hotel and Tower Chicago had the word "RESIST" across the top, along with the Greenpeace logo.
The word "defend" was superimposed over a picture of the Earth on the enormous flag.
News reports said seven people were arrested and the banner taken down.
The Chicago Tribune newspaper said that the banner appeared to hang from cables strung from a terrace restaurant and bar at the high-rise.