The female was found in the Llandovery area last month, struggling with a fever and dehydration.
She was rehabilitated by RSPCA staff at a specialist wildlife facility in Taunton, Somerset.
Her weight also increased from 2.8kg to 4.8kg by the time she was released near to where she was found.
An otter has been returned to the wild in Carmarthenshire after being found curled up in a puddle of water from a pressure hose.