The rower, who won the Women's Double Sculls along with Sophie Hosking on Friday (August 3), received a golden post box in her hometown of Stockton to mark her triumph.
However, the landmark has already been vandalised since getting a makeover on Sunday (August 5), with nicknames scratched into the surface.
"We were made aware of the Olympic gold postbox having been vandalised by a member of the public," a Cleveland Police spokesperson told ITV News.
"Two girls aged 11 and 15 from outside of the area are believed to be responsible and arrangements are being made by police to speak with them."
Jessica Ennis's gold post box in Sheffield to celebrate her Heptathlon victory was also vandalised earlier this week. The words "Go Jess" had been written on the structure within 24 hours of it being painted.
Royal Mail applied a new coat to Ennis's post box as "a point of pride" and said that they were "extremely disappointed" to have seen it defaced.