It comes after one was found hiding under a water tank in a garden at Kirton Holme, near Boston, Lincolnshire, over the Easter weekend.
Inspector Becky Harper said: "While he is very cute, we'd like to stress that raccoon dogs don't make good pets."
"They are wild animals and we would strongly discourage people from buying or keeping one," she said.
"They need a great deal of space, and their needs cannot be met in a typical domestic environment."
More on this and other local stories from across Lincolnshire
The RSPCA said it had dealt with a number of call-outs in recent years to stray pet raccoon dogs that have escaped, or been deliberately released to the wild.
Ms Harper added that the animals posed a high level of threat to our native wildlife.
The raccoon dog - now nicknamed Cedric by RSPCA staff - is being cared for at a specialist centre after efforts to trace its owners failed.
He is due to be re-homed to a specialist keeper.

Feminist groups were joined outside Belfast City Hall by people supporting gay rights, immigrants and the disabled.
Organisers said the aim was to highlight women's rights.
The protest comes after a series of controversial comments by President Trump during his election campaign.
Protesters said they believe the rights of many Americans are under threat from the new administration.
They said they were overwhelmed by the number of people who took part.
The events draw inspiration from the Women's March on Washington, which also took place on Saturday and attracted crowds of 200,000 people.
Almost 700 so-called sister marches took place on Saturday across the globe. Protests by women have already taken place in Australia, New Zealand and Japan.
In the UK, rallies took place in London, Cardiff, Edinburgh, Lancaster, Leeds, Liverpool, Manchester, Bristol and Shipley.
London organisers announced on stage that between 80,000 and 100,000 men, women and children had taken part in the rally.
However, police have not given an independent estimate and the BBC has no way of verifying the figure.

His solicitor Julia Cooper has asked Surrey Police to explain a statement that he had confessed to the murder.
Bellfield, 47, was given a whole-life prison sentence in June 2011 for murdering the schoolgirl.
Surrey Police confirmed it had received a letter from Bellfield's solicitors but said it was standing by its statement on the confession.
Milly was kidnapped while on her way from school to her home in Walton-on-Thames, Surrey, in March 2002.
Ms Cooper said Bellfield, now known as Yusuf Rahim, believed a "covert tape" may have been used to record his response to a police interview in prison.
She said "my client denies giving a confession".
She has written to Surrey Police requesting the tape recording and notes from the meeting.
Ms Cooper said Bellfield contacted her very soon after Surrey Police distributed its press statement relating to the Dowler case.
She said Bellfield denies all the crimes for which he has been convicted.
Last month, Surrey Police revealed Bellfield had made the admission during an investigation into whether he had an accomplice involved in the abduction and rape, but not murder, of Milly.
The force said it stood by its original statement.
A police source speaking to the BBC would not comment on how the admission was recorded.
Milly's body was found 25 miles away from where she was kidnapped, in Yateley Heath, Hampshire.
Experts could not say how she died.
Bellfield was found guilty of abducting and murdering the teenager following a trial at the Old Bailey where a judge described him as a "cruel and pitiless killer".
He was already in jail for the murders of Amelie Delagrange, 22, and Marsha McDonnell, 19, and the attempted murder of Kate Sheedy, 18, when he went on trial accused of killing Milly.
He lived 50 yards from where Milly vanished but did not become a suspect until he was arrested by police in London for the other crimes in 2004.
On Wednesday, the Dowler family revealed harrowing details of Milly's final hours, saying they had been made aware of her suffering last year after Bellfield had spoken to police.
The RSPCA has issued a warning against what it says is a growing trend for keeping raccoon dogs as pets.
[NEXT_CONCEPT]
Hundreds gathered for a Women's March in Belfast on Saturday as part of an international protest on the first day of Donald Trump's presidency.
[NEXT_CONCEPT]
Serial killer Levi Bellfield has denied confessing to the abduction, rape and killing of 13-year-old Milly Dowler.