The six-month old female was weak and undernourished when Moira Banks came across her in her home in the Brecon Beacons, Wales.
Paul Oaten, wildlife supervisor at West Hatch, said: "It is not unusual for otters to go into houses when they are in as bad a condition as this little one was."She was very lucky to find her way into the house of someone so quick-thinking and it is great that she made enough progress to be released back where she belongs nearly a year to the day after she was found."
Ms Banks said: "It was great to see the otter released - she was so much livelier and feistier after the year of RSPCA care.
"Co-incidentally, the morning before we went to see her release my husband saw another otter, as he walked the dogs - just six feet away from him.
"They are the only two otters either of us have seen in 20 years of living here. We are hoping that the first was a male, who might come across the female, and there may soon be lots of babies."