French President Nicolas Sarkozy, who was present at the match with his son has said what was done by these football fans was "inadmissible".
The banner was held up for a short while during the second half of the match, and the wording on it is a reference to the title of a recent French film, Welcome to the land of the Ch'tis.
President Nicolas Sarkozy added that he was "a dismayed witness to the unfurling of a hateful banner."
Both Paris St Germain and Lens football clubs have made legal complaints about the incident, and so has the French Football League.
Paris St Germain has been known to have had trouble from their fans in the past.
If French police catch the people behind this they could be charged with inciting hatred or violence, and could face imprisonment.
