Written as an article in the fictional Daily Prophet newspaper under the headline "Dumbledore's Army Reunites at Quidditch World Cup Final", the 1,500-word story also features the two other main characters in the Harry Potter trio, Hermione Granger and Ron Weasley - now married to each other.
Potter, Rowling writes, has clung to some aspects of his youth:
"About to turn 34, there are a couple of threads of silver in the famous ... black hair, but he continues to wear the distinctive round glasses that some might say are better suited to a style-deficient 12-year-old."
As the trio watch the final of the Quidditch contest - the celebrated Hogwarts sport involving flying broomsticks and enchanted balls - Rita Skeeter, a vicious journalist from previous novels, discusses their lives in a gossip column.
The seven Harry Potter novels, the first of which was published in 1997, became the best-selling book series ever and has been adapted into a multibillion-dollar film franchise.
The new story, on Rowling's website Pottermore, an interactive site for fans begun in 2011, is here.