At present, any British athlete that wins a gold medal at the ongoing Games has a postbox coated in the same colour in their home town.
On Sunday morning, the paint was applied to one in Sheffield, where Ennis was born.
However, someone has since vandalised the box by writing the words 'Go Jess' across the front of it.
"We are all so proud of Jess and her fantastic success, which is not only a great achievement for her and her team but also is a huge achievement for the city," said the leader of the Sheffield city council Julie Dore.
"This is our moment to shine so what a shame it is that someone has chosen to tarnish this by vandalising a postbox painted gold in honour of our local hero. I hope they feel thoroughly ashamed of themselves."
The Royal Mail have confirmed that they will be sending out an engineer to repair the damage.