The company published an ad on its Facebook page on Sunday showing the nude man with the headline "Clearly, not everyone knows that we have swimming trunks from €9.99."
The ad shows the same blurry man but this time sporting a pair of snug black trunks to protect his modesty.
The man's appearance in the background of a catalogue spread for children's clothes caused much hilarity on the internet on Wednesday.
Within hours of the discovery the man was being added to other images, including the first steps of man on the moon and the Back to the Future film poster.
La Redoute was not so amused and issued apologies immediately.
"An internal inquiry has been opened to determine how the error happened," said a spokeswoman for the company.